Stimulus selection via differential response latencies in visual cortical area V4

Any given region of the cerebral cortex gets multiple inputs, and how these inputs are combined or selected is a key component of cortical function. Experiments in brain slices or other reduced preparations have shown that excitatory inputs to cortex produce a delayed feed-forward inhibition, which suggests that the relative timing of inputs at the scale of tens of milliseconds is crucial to cortical operation. Other mechanisms, such as synaptic depression and feedback inhibition, have also been shown to produce strong effects on this timescale. Thus, the relative timing of inputs should be fundamental in determining how a given region of cortex selects or combines its inputs. A rhesus monkey (Macaca mulatta) was trained to fixate on a spot of light for juice reward. Isolated single units in visual cortical area V4 were recorded using standard microelectrode techniques. Two visual stimuli were positioned such that each alone elicited a strong response. The stimuli were presented both separately and in combination, and their contrast and relative onset timing were varied. In general, the response of each neuron to two stimuli was locked to the response to that single stimulus that produced the shortest latency. A partial exception was that the responses to low-contrast stimuli were often less effective at suppressing later-arriving responses to high-contrast stimuli. The presentation of two stimuli in the receptive field of a visual cortical neuron is proposed as a model system for how changes in the relative timing of inputs affect cortical function in the intact system.     

Activation and deactivation in response to visual stimulation in the occipital cortex of 6-month-old human infants

In an infant's developing cortex, the explanation for the mechanisms underlying the activations and deactivations in response to visual stimuli remains controversial. While previous near-infrared spectroscopy (NIRS) studies in awake infants have demonstrated cortical activations in response to meaningful/attractive visual stimuli, functional magnetic resonance imaging (fMRI) studies performed on sleeping infants showed negative blood oxygenation level-dependent (BOLD) responses to high-luminance unpatterned stimulations, such as a photic stimulation. To examine the effect of the characteristics of visual stimuli on cortical processing in awake infants, we measured cortical hemodynamic responses in 6-month-old infants during the presentation of a high-luminance unpatterned stimulus by using a NIRS system with 94 measurement channels. Results from 35 infants showed dissociated cortical responses between the occipital region and the other parts of the cortex, including the temporal and prefrontal regions. Although the visual stimulus produced sustained increases in oxygenated hemoglobin (oxy-Hb) signals in the temporal and prefrontal regions, it produced a transient increase in oxy-Hb signals followed by a salient decrease in oxy-Hb signals during a trial in a focal region of the occipital visual region. This suggests that the deactivation of the occipital visual region in response to visual stimulation is not a phenomenon that occurs only in the sleeping state, but that a high-luminance unpatterned stimulus can induce deactivation even in the awake infants.     

Emotional and behavioral correlates of the anterior cingulate cortex during associative learning in rats.

Neuronal activity was recorded from the anterior cingulate cortex of behaving rats during discrimination and learning of conditioned stimuli associated with or without reinforcements. The rats were trained to lick a protruding spout just after a conditioned stimulus to obtain reward (intracranial self-stimulation or sucrose solution) or to avoid aversion. The conditioned stimuli included both elemental (auditory or visual stimuli) and configural (simultaneous presentation of auditory and visual stimuli predicting reward outcome opposite to that predicted by each stimulus presented alone) stimuli. Of the 62 anterior cingulate neurons responding during the task, 38 and four responded differentially and non-differentially to the conditioned stimuli (conditioned stimulus-related neurons), respectively. Of the 38 differential conditioned stimulus-related neurons, 33 displayed excitatory (n = 10) and inhibitory (n = 23) responses selectively to the conditioned stimuli predicting reward. These excitatory and inhibitory differential conditioned stimulus-related neurons were located mainly in the cingulate cortex areas 1 and 3 of the rostral and ventral parts of the anterior cingulate cortex, respectively. The remaining 20 neurons responded mainly during intracranial self-stimulation and/or ingestion of sucrose (ingestion/intracranial self-stimulation-related neurons). Increase in activity of the ingestion/intracranial self-stimulation-related neurons was correlated to the first lick to obtain rewards during the task, suggesting that the activity reflected some aspects of motor functions for learned instrumental behaviors. These ingestion/intracranial self-stimulation-related neurons were located sparsely in cingulate cortex area 1 of the rostral part of the anterior cingulate cortex and densely in frontal area 2 of the caudal and dorsal parts of the anterior cingulate cortex. Analysis by the multidimensional scaling of responses of 38 differential conditioned stimulus-related neurons indicated that the anterior cingulate cortex categorized the conditioned stimuli into three groups based on reward contingency, regardless of the physical characteristics of the stimuli, in a two-dimensional space; the three conditioned (two elemental and one configural) stimuli predicting sucrose solution, the three conditioned (two elemental and one configural) stimuli predicting no reward, and the lone conditioned stimulus predicting intracranial self-stimulation. The results suggest that the anterior cingulate cortex is organized topographically; stimulus attributes predicting reward or no reward are represented in the rostral and ventral parts of the anterior cingulate cortex, while the caudal and dorsal parts of the anterior cingulate cortex are related to execution of learned instrumental behaviors. These results are in line with recent neuropsychological studies suggesting that the rostral part of the anterior cingulate cortex plays a crucial role in socio-emotional behaviors by assigning a positive or negative value to future outcomes.     

Learning increases stimulus salience in anterior inferior temporal cortex of the macaque.

With experience, an object can become behaviorally relevant and thereby quickly attract our interest when presented in a visual scene. A likely site of these learning effects is anterior inferior temporal (aIT) cortex, where neurons are thought to participate in the filtering of irrelevant information out of complex visual displays. We trained monkeys to saccade consistently to one of two pictures in an array, in return for a reward. The array was constructed by pairing two stimuli, one of which elicited a good response from the cell when presented alone ("good" stimulus) and the other of which elicited a poor response ("poor" stimulus). The activity of aIT cells was recorded while monkeys learned to saccade to either the good or poor stimulus in the array. We found that neuronal responses to the array were greater (before the saccade occurred) when training reinforced a saccade to the good stimulus than when training reinforced a saccade to the poor stimulus. This difference was not present on incorrect trials, i.e., when saccades to the incorrect stimulus were made. Thus the difference in activity was correlated with performance. The response difference grew over the course of the recording session, in parallel with the improvement in performance. The response difference was not preceded by a difference in the baseline activity of the cells, unlike what was found in studies of cued visual search and working memory in aIT cortex. Furthermore, we found similar effects in a version of the task in which any of 10 possible pairs of stimuli, prelearned before the recording session, could appear on a given trial, thereby precluding a working memory strategy. The results suggest that increasing the behavioral significance of a stimulus through training alters the neural representation of that stimulus in aIT cortex. As a result, neurons responding to features of the relevant stimulus may suppress neurons responding to features of irrelevant stimuli.     

Dynamics of response to perceptual pop-out stimuli in macaque V1

Contextual modulation due to feature contrast between the receptive field and surrounding region has been reported for numerous stimuli in primary visual cortex. One type of this modulation, iso-orientation surround suppression, has been studied extensively. The degree to which surround suppression is related to other forms of contextual modulation remains unknown. We used shape-from-shading stimuli in a field of distractors to test the latency and magnitude of contextual modulation to a stimulus that cannot be distinguished with an orientation-selective mechanism. This stimulus configuration readily elicits perceptual pop-out in human observers and induces a long-latency contextual modulation response in neurons in macaque early visual cortex. We found that animals trained to detect the location of a pop-out stimulus were better at finding a sphere that appeared to be lit from below in the presence of distractors that were lit from above. Furthermore, neuronal responses were stronger and had shorter latency in the condition where behavioral performance was best. This asymmetry is compatible with earlier psychophysical findings in human observers. In the population of V1 neurons, the latency of the contextual modulation response is 145 ms on average (ranging from 70 to 230 ms). This is much longer than the latency for iso-orientation surround suppression, indicating that the underlying circuitry is distinct. Our results support the idea that a feature-specific feedback signal generates the pop-out responses we observe and suggest that V1 neurons actively participate in the computation of perceptual salience.     

Neuronal responses from beyond the classic receptive field in V1 of alert monkeys

Responses of primary visual cortex (V1) neurons to stimuli inside the classic receptive field (CRF) can be modulated by stimuli outside the CRF. We recently reported that responses of most V1 neurons to a line in the CRF center are inhibited by large surround-stimuli and that this modulation is stimulus selective. Here we report that a significant proportion of V1 neurons in alert monkeys respond directly to stimuli outside the CRF with very long latency and much reduced selectivity. When surround stimuli are presented alone, three response patterns can be distinguished in 153 single- or multiunits tested: (1) 31.4% have no significant response; (2) 50.3% show excitatory responses that are significantly higher than spontaneous activity. The average latency of these responses is about 145 ms, 2–3 times longer than center responses; (3) 18.3% show suppressed spontaneous activity after stimulus onset. The direct surround responses are found to be only weakly selective for the orientation of contextual lines, and not selective for other contextual patterns tested. While the outburst of responses to stimuli within the CRF is not affected by reducing stimulus duration from 500 ms to 50 ms, late excitatory surround responses are virtually eliminated. We propose that the late excitatory surround responses to extra-CRF stimulation alone are the reflection of feedback from higher cortical areas and may contribute to reduced contextual inhibition of cells in V1. This could play a role in figure-ground segregation. Electronic Publication     

Spatial and cross-modal attention alter responses to unattended sensory information in early visual and auditory human cortex

Attending to a visual or auditory stimulus often requires irrelevant information to be filtered out, both within the modality attended and in other modalities. For example, attentively listening to a phone conversation can diminish our ability to detect visual events. We used functional magnetic resonance imaging (fMRI) to examine brain responses to visual and auditory stimuli while subjects attended visual or auditory information. Although early cortical areas are traditionally considered unimodal, we found that brain responses to the same ignored information depended on the modality attended. In early visual area V1, responses to ignored visual stimuli were weaker when attending to another visual stimulus, compared with attending to an auditory stimulus. The opposite was true in more central visual area MT+, where responses to ignored visual stimuli were weaker when attending to an auditory stimulus. Furthermore, fMRI responses to the same ignored visual information depended on the location of the auditory stimulus, with stronger responses when the attended auditory stimulus shared the same side of space as the ignored visual stimulus. In early auditory cortex, responses to ignored auditory stimuli were weaker when attending a visual stimulus. A simple parameterization of our data can describe the effects of redirecting attention across space within the same modality (spatial attention) or across modalities (cross-modal attention), and the influence of spatial attention across modalities (cross-modal spatial attention). Our results suggest that the representation of unattended information depends on whether attention is directed to another stimulus in the same modality or the same region of space.     

Both striate cortex and superior colliculus contribute to visual properties of neurons in superior temporal polysensory area of macaque monkey

Although the tectofugal system projects to the primate cerebral cortex by way of the pulvinar, previous studies have failed to find any physiological evidence that the superior colliculus influences visual activity in the cortex. We studied the relative contributions of the tectofugal and geniculostriate systems to the visual properties of neurons in the superior temporal polysensory area (STP) by comparing the effects of unilateral removal of striate cortex, the superior colliculus, or of both structures. In the intact monkey, STP neurons have large, bilateral receptive fields. Complete unilateral removal of striate cortex did not eliminate visual responses of STP neurons in the contralateral visual hemifield; rather, nearly half the cells still responded to visual stimuli in the hemifield contralateral to the lesion. Thus the visual properties of STP neurons are not completely dependent on the geniculostriate system. Unilateral striate lesions did affect the response properties of STP neurons in three ways. Whereas most STP neurons in the intact monkey respond similarly to stimuli in the two visual hemifields, responses to stimuli in the hemifield contralateral to the striate lesion were usually weaker than responses in the ipsilateral hemifield. Whereas the responses of many STP neurons in the intact monkey were selective for the direction of stimulus motion or for stimulus form, responses in the hemifield contralateral to the striate lesion were not selective for either motion or form. Whereas the median receptive field in the intact monkey extended 80 degrees into the contralateral visual field, the receptive fields of cells with responses in the contralateral field that survived the striate lesions had a median border that extended only 50 degrees into the contralateral visual field. Removal of both striate cortex and the superior colliculus in the same hemisphere abolished the responses of STP neurons to visual stimuli in the hemifield contralateral to the combined lesion. Nearly 80% of the cells still responded to visual stimuli in the hemifield ipsilateral to the lesion. Unilateral removal of the superior colliculus alone had only small effects on visual responses in STP. Receptive-field size and visual response strength were slightly reduced in the hemifield contralateral to the collicular lesion. As in the intact monkey, selectivity for stimulus motion or form were similar in the two visual hemifields. We conclude that both striate cortex and the superior colliculus contribute to the visual responses of STP neurons. Striate cortex is crucial for the movement and stimulus specificity of neurons in STP.(ABSTRACT TRUNCATED AT 400 WORDS)     

Suppressive effects of receptive field surround on neuronal activity in the cat primary visual cortex

Effects of sinusoidal grating stimulus presented outside the classical receptive field (CRF) on neuronal responses were studied in the primary visual cortex of anaesthetized cats. Among 101 cells electrophysiologically recorded, the predominant effect of the stimulus in the receptive field surround (SRF) was the suppression of responses to the CRF stimulation, and the SRF grating suppressed them up to 56% of the responses (44% suppression) to the CRF stimulus alone. The strong suppression was observed more often in layer II/III cells than in other layers and in complex cells more often than in simple cells. The modulatory effects by SRF stimulus might be enhanced by the cortical recurrent excitation particularly in the superficial layers. We also examined whether the modulation by the surround grating exhibits a differential effect according to the presence or absence of figure-ground segregation in the stimulus configuration. For this purpose, effects of stimulus configuration with orientation-, direction-contrast or relative spatial phase difference between CRF and SRF stimuli (figure-ground segregated configuration) were compared with those of uniform configuration of stimulus (non-segregated configuration). There was a population of cells, which exhibited significantly stronger suppression with non-segregated configuration than with figure-ground segregated configuration. Such differential modulation of response by the SRF stimulus in the primary visual cortex is a possible basis of perceptual figure-ground segregation.     

Assessment of stimulus-induced changes in human V1 visual field maps

Visual cortex contains a set of field maps in which nearby scene points are represented in the responses of nearby neurons. We tested a recent hypothesis that the visual field map in primary visual cortex (V1) is dynamic, changing in response to stimulus motion direction. The original experimental report replicates, but further experimental and analytical investigations do not support, the interpretation of the results. The V1 map remains invariant when measured using stimuli moving in different directions. The measurements can be explained by small and systematic response amplitude differences that arise when probing with stimuli moving in different directions.     

GABA-inactivation attenuates colinear facilitation in cat primary visual cortex

Neurons in primary visual cortex (V1) respond preferentially to stimuli of a particular orientation falling within a circumscribed region of visual space known as their receptive field (RF). However, the response to an optimally oriented stimulus presented within the RF can be enhanced by the simultaneous presentation of co-oriented, co-linearly aligned flank stimuli falling outside the RF which, when presented alone, fail to activate the cell. This type of contextual effect, termed colinear facilitation, presumably forms the physiological substrate for the integration of the line elements of a contour and the perceptual saliency of a contour in a complex environment. Here we show that colinear facilitation in single cells of cat area V1 can be substantially reduced or abolished by focal inactivation of laterally remote cells in the same area which respond strongly to the co-oriented, colinear flank stimulus inducing the facilitatory effect. The results provide evidence that horizontal intrinsic connections between cells with co-oriented and co-linearly aligned RFs make a major contribution to colinear facilitation in V1. They imply that the neuronal circuitry underlying contour integration and saliency is already present at the earliest stage of visual cortical information processing.     

Visual response properties of neurons in the parahippocampal cortex of monkeys

We examined visual response properties of single neurons in the parahippocampal (PH) cortex of alert monkeys using various visual stimuli (bars, geometrical shapes such as a circle, and images such as a human face) while the monkey fixated a spot for a juice reward. Of the investigated PH neurons 104 of 359 (29%) were found to be visually responsive. The investigation was focused on spatial and object aspects of visual processing. We investigated a visual receptive field (RF) property and a direction selectivity for a moving bar with respect to spatial processing. For half of these PH neurons (53%), the optimal stimulus position, where a visual stimulus elicited the maximal response, located peripherally, that is, with an eccentricity of more than 10 deg. More than 20% of these PH neurons had an RF that does not include the center of gaze. There were neurons in the PH cortex that appeared to convey motion signals. In addition, some PH neurons showed eye-position-dependent activity. With respect to object processing, we investigated selectivities for images, geographical shapes, orientations of a bar, and colors. For comparison purposes, we also examined responses of perirhinal (PR) neurons. PH neurons showed selective responses to these stimuli, but PR neurons were found to be more selective for images than PH neurons. These results suggest that the PH cortex is involved in both spatial and object processing, but less involved than the PR cortex in processing of complex images.     

Functional imaging of the human lateral geniculate nucleus and pulvinar

In the human brain, little is known about the functional anatomy and response properties of subcortical nuclei containing visual maps such as the lateral geniculate nucleus (LGN) and the pulvinar. Using functional magnetic resonance imaging (fMRI) at 3 tesla (T), collective responses of neural populations in the LGN were measured as a function of stimulus contrast and flicker reversal rate and compared with those obtained in visual cortex. Flickering checkerboard stimuli presented in alternation to the right and left hemifields reliably activated the LGN. The peak of the LGN activation was found to be on average within +/-2 mm of the anatomical location of the LGN, as identified on high-resolution structural images. In all visual areas except the middle temporal (MT), fMRI responses increased monotonically with stimulus contrast. In the LGN, the dynamic response range of the contrast function was larger and contrast gain was lower than in the cortex. Contrast sensitivity was lowest in the LGN and V1 and increased gradually in extrastriate cortex. In area MT, responses were saturated at 4% contrast. Response modulation by changes in flicker rate was similar in the LGN and V1 and occurred mainly in the frequency range between 0.5 and 7.5 Hz; in contrast, in extrastriate areas V4, V3A, and MT, responses were modulated mainly in the frequency range between 7.5 and 20 Hz. In the human pulvinar, no activations were obtained with the experimental designs used to probe response properties of the LGN. However, regions in the mediodorsal right and left pulvinar were found to be consistently activated by bilaterally presented flickering checkerboard stimuli, when subjects attended to the stimuli. Taken together, our results demonstrate that fMRI at 3 T can be used effectively to study thalamocortical circuits in the human brain.     

Cholinergic modulation of response gain in the primary visual cortex of the macaque

ACh modulates neuronal activity throughout the cerebral cortex, including the primary visual cortex (V1). However, a number of issues regarding this modulation remain unknown, such as the effect and its function and the receptor subtypes involved. To address these issues, we combined extracellular single-unit recordings and microiontophoretic administration of ACh and measured V1 neuronal responses to drifting sinusoidal grating stimuli in anesthetized macaque monkeys. ACh was found to have mostly facilitatory effects on the visual responses, although some cases of suppressive effects were also seen. To assess the functional role of ACh, we further examined how ACh modulates the stimulus contrast-response function, finding that the response gain increased with the facilitatory effect. The response facilitation was completely or strongly blocked by atropine (At), a muscarinic ACh receptor (mAChR) antagonist, in almost all neurons (96% of cells), whereas any residual effect after At administration was fully removed by mecamylamine, a nicotinic AChR (nAChR) antagonist, suggesting a predominant role for mAChRs in this mechanism. Furthermore, we found no laminar distribution bias for the facilitatory modulation, although the relative contribution of mAChRs was smaller in layer 4C than in other layers. The suppressive effect was blocked completely by At. These results demonstrate that ACh plays an important role in visual information processing in V1 by controlling the response gain via mAChRs across all cortical layers and via nAChRs, mainly in layer 4C.     

Orientation sensitivity of the cat assessed from evoked potentials: central and peripheral contributions

To estimate contour-orientation sensitivity of the cat and the degree to which precortical processing contributes to such estimates, the amplitude of visually evoked potentials (VEP) recorded from the visual cortex of cats in response to a visual stimulus (S2) presented at various intervals after presentation of another visual stimulus (S1) was measured under several conditions. Recordings were made when both stimuli were presented through one eye (monoptic condition) or when S1 was presented to one eye and S2 to the other (dichoptic condition). In some experiments, simultaneous recordings were made from the optic tract and visual cortex. The stimuli were pairs of sinusoidal gratings with a spatial frequency of 0.5 cycles/deg and of various orientations. Each stimulus was presented by stepping the grating contrast from 0.0 (adapting field) to 0.5 for 50 ms. The intervals between the presentation of the two test stimuli (S1 and S2) was varied from 0 to 1,550 ms, and on different trials the orientation of the S2 grating relative to that of S1 was varied from 0 to 90 degrees. Results showed that under monoptic conditions, the VEP to the second stimulus (S2) was reduced by presentation of the first stimulus (S1) when the interstimulus interval was less than 200 ms, whereas under dichoptic conditions, the response to S2 was reduced with interstimulus intervals less than 75 ms. The response reduction was always in a forward direction (e.g., reduced S2 response), increased in magnitude with decreases in the interstimulus interval, and was larger under monoptic conditions than under dichoptic conditions. The response reduction produced monoptically was orientation selective in that it was greatest when the orientation of S1 and S2 was the same, and it recovered by half when the orientation differed by 6 to 15 degrees (orientation half-band pass). In some cortical recordings, the orientation-selective response reduction was superimposed on a response reduction that was not selective for S2 orientation. Stimultaneous recording in the optic tract also showed a response reduction of S2 response that was not orientation selective, suggesting that precortical neural elements contribute to the cortical VEP. With dichoptic stimulus presentation an orientation-nonspecific response reduction was obtained. We hypothesized that binocular inhibitory effects, resulting from disparate retinal input, produced this surprising finding. The results demonstrate that the VEP recorded at the cortex can be used to estimate orientation sensitivity, but that response interactions in peripheral (precortical) neural elements can contribute to such estimates.     

Dynamic properties of recurrent inhibition in primary visual cortex: contrast and orientation dependence of contextual effects

A fundamental feature of neural circuitry in the primary visual cortex (V1) is the existence of recurrent excitatory connections between spiny neurons, recurrent inhibitory connections between smooth neurons, and local connections between excitatory and inhibitory neurons. We modeled the dynamic behavior of intermixed excitatory and inhibitory populations of cells in V1 that receive input from the classical receptive field (the receptive field center) through feedforward thalamocortical afferents, as well as input from outside the classical receptive field (the receptive field surround) via long-range intracortical connections. A counterintuitive result is that the response of oriented cells can be facilitated beyond optimal levels when the surround stimulus is cross-oriented with respect to the center and suppressed when the surround stimulus is iso-oriented. This effect is primarily due to changes in recurrent inhibition within a local circuit. Cross-oriented surround stimulation leads to a reduction of presynaptic inhibition and a supraoptimal response, whereas iso-oriented surround stimulation has the opposite effect. This mechanism is used to explain the orientation and contrast dependence of contextual interactions in primary visual cortex: responses to a center stimulus can be both strongly suppressed and supraoptimally facilitated as a function of surround orientation, and these effects diminish as stimulus contrast decreases.     

The effect of varying stimulus intensity on NMDA-receptor activity in cat visual cortex

1. A study was made of the relative contribution of N-methyl-D-aspartate (NMDA) and non-NMDA receptors to the visual responses of cells in different layers of the cat visual cortex at different levels of excitatory drive (which was varied by altering the stimulus contrast). 2. Receptive fields were mapped for 121 cells in area 17 of cat cortex. Cells were characterized to determine the optimal visual stimulus, the brightness of which was then varied relative to background luminance to construct a contrast-response (C-R) curve for each cell. Curves were made during control conditions and during application of agonists (NMDA and quisqualate) and/or antagonists [(D)-2-amino-5-phosphonovaleric acid (D-APV) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)] to examine the excitatory amino acid components of the visual response. 3. Threshold responses were obtained with stimuli between 1/60 and 1.8 X background luminance. The cell response, measured by firing rate, was linearly related to stimulus contrast over 1-2 decades and saturated at higher contrasts. 4. Application of APV reduced the slope of the linear portion of the C-R curve for cells located in layers II and III (average reduction, 59% of control). APV did not decrease the threshold to stimulation. The "just suprathreshold" responses to stimulation were reduced by the same proportion as the saturation responses for individual cells. The principal effect was therefore to reduce the gain of the C-R curve in these layers. 5. Application of APV reduced the spontaneous activity of cells located in layers IV, V, and VI with little if any effect on the gain of the C-R curve. This suggests a tonic background level of NMDA-receptor activation in these layers, which is not directly related to the visual response. 6. Low levels of NMDA increased the gain of the C-R curve in layers II/III and V/VI. On the other hand, low levels of quisqualate increased the overall level of firing without affecting the gain of the C-R curve. NMDA did not increase the gain of the curve in layer IV. 7. These experiments show that visual stimuli that produce just suprathreshold responses activate NMDA receptors. The degree of activation is proportionally the same for small responses and large responses for an individual cell. Rather than finding a threshold for NMDA-receptor activation, a continuous range of NMDA-receptor influence was observed over the entire response range.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparative study on the offset responses of simple cells and complex cells in the primary visual cortex of the cat

Simple and complex cells are two basic and distinct functional types of neurons in the mammalian primary visual cortex. Here, we studied the onset response and the offset response of simple and complex cells to a flashing visual stimulus in the cat's area 17. Compared with simple cells, complex cells exhibited greater similarity between the onset and offset responses in peak latency. For simple cells, onset response had greater peak amplitude and signal-to-noise ratio than offset response, and for complex cells, vice versa. For both types of cortical cells, the amplitude of offset responses increased with stimulus duration within 100 ms significantly, while the onset response did not. However, to elicit a detectable offset response, complex cells tended to require shorter stimulus duration than simple cells did. In regard to the similarity of psychophysical data, these results suggest that the rebound offset response of cortical cells to disappearance of a visual pattern might be correlated to visual persistence in humans.     

Neural responses in motor cortex and area 7a to real and apparent motion

The neural activity in area 7a and the arm area of motor cortex was recorded while real or path-guided apparent motion stimuli were presented to behaving monkeys in the absence of a motor response. A smooth stimulus motion was produced in the real motion condition, whereas in the apparent motion condition five stimuli were flashed successively at the vertices of a regular pentagon. The stimuli moved along a low contrast circular path with one of five speeds (180–540 deg/s). We found strong neural responses to real and apparent motion in area 7a and motor cortex. In the motor cortex, a substantial population of neurons showed a selective response to real moving stimuli in the absence of a motor response. This activity was modulated in some cases by the stimulus speed, and some of the neurons showed a response during a particular part of the circular trajectory of the stimulus; the preferred stimulus angular locations were evenly distributed across this neuronal ensemble. It is likely that these neural signals are continuously available to the motor cortex in order to generate responses that demand immediate action. In area 7a, two overlapping populations of neurons were observed. The first comprised cells the activity of which was tuned to the angular location of a circularly moving stimulus in the real motion condition. These cells also responded to apparent motion at high stimulus speeds. A visual receptive field analysis showed that the angular tuning in most of the area 7a neurons did not depend on the spatial location of the stimulus in relation to their receptive field. The second population was selective to apparent moving stimuli and showed a periodic entrainment of activation with the period of the inter-stimulus interval of the flashing dots. Both the angular location and the inter-stimulus interval neural signals can be used to generate precise behavioral responses towards real or apparent moving stimuli.     

Dark-reared cats: unresponsive cells become visually responsive with microiontophoresis of an excitatory amino acid

Summary The visual system of kittens reared in total darkness is grossly abnormal. Although estimates vary, substantial proportions of cells in the visual cortex of these animals are unresponsive to visual stimulation. Additional cells are weakly responsive or erratic. We have considered the possibility that these neurons receive subthreshold input which might be evident if an excitatory neurochemical agent is applied during extracellular recording with a microelectrode. To test this notion, we have recorded from cells in the striate cortex of dark-reared kittens during microiontophoretic application of an excitatory amino acid, DL-homocysteate (DLH). Using this technique, we find that virtually all cells in the visual cortex of dark-reared kittens are responsive to visual stimulation. Prior to application of DLH, 27% of the cells were unresponsive to visual stimuli. Following iontophoresis of DLH, half of these cells responded with excitatory discharge to visual stimuli and the other half exhibited an inhibitory response in that the elevated maintained activity was suppressed during presentation of a visual stimulus. Additional cells from these animals, which were initially visually responsive, were also studied. For some of these units, responses were weak prior to administration of DLH and we were able to obtain a more clear estimate of selectivity for stimulus orientation during microiontophoresis of the drug. In these cases, and for the few cells which were initially responsive and orientation selective, we observed no major differences in selectivity before and after DLH application.On leave from Universidade Federal do Rio de Janeiro, Centro de Ciencias da Saude, Dept. de Farmacologia, Bloco J, Ilha Universitaria, 21941, Rio de Janeiro, Brasil