Perceptual and Cognitive Visual Functions of Parietal and Temporal Cortices in the Cat

We used reversible cooling deactivation to compare the functionsof cortices lining the middle suprasylvian (MS) sulcus and formingthe ventral portion of the posterior suprasylvian (vPS) gyrus.A battery of attentional, motion and mnemonic processing taskswere used and performance was examined during deactivation ofeach region. The results show a clear dissociation of functions.Deactivation of MS cortex resulted in profound deficits on avisual orienting task and on the discrimination of directionof motion, whereas deactivation of vPS cortex severely impairedboth retention and learning of novel and overlearned objectdiscriminations. In addition, deactivation of either MS or vPScortex impaired discrimination of learned patterns when componentsof the patterns were in motion, whereas only deactivation ofvPS cortex impaired the discrimination when all components werestatic. Together, these results show that a region of parietalcortex contributes to the processing of visual motion and toattentional processes, whereas a region of temporal cortex contributesto the learning and recognition of three-dimensional objectsand two-dimensional patterns. This functional dissociation islinked to differences in underlying visual pathways, which havemany features in common with the parietal and temporal visualprocessing streams previously identified in monkeys and humans.Furthermore, the broad similarity in neural operations carriedout in parietal and temporal cortices of cats, monkeys and humanssuggests the existence of a common plan for cortical processingmachinery within mammals with well developed cerebral cortices.     

Translaminar differentiation of visually guided behaviors revealed by restricted cerebral cooling deactivation

The purpose of the present study was to test the hypothesis that superficial and deep layers within a single cerebral region influence cerebral functions and behaviors in different ways. For this test, we selected posterior middle suprasylvian (pMS) sulcal cortex of the cat, a suspected homolog of the area V5 complex of primates, because the region has been implicated in several visually guided behaviors. Cats were trained on three tasks: (1) discrimination of direction of motion; (2) discrimination of static patterns partially obscured by static or moving masks; and (3) visual detection and orienting. Cooling of cryoloops in contact with pMS sulcal cortex to 8+/-1 degrees C selectively and completely impaired performance on the two motion discrimination tasks (1 and 2), while leaving the detection and orienting task (task 3) unimpaired. Further cooling to 3 degrees C resulted in an additional complete impairment of task 3. The 8 degrees C temperature resulted in silencing of neuronal activity in the supragranular layers (I-III) and the 3 degrees C temperature silenced activity throughout the thickness of pMS sulcal cortex. The variation in behavioral performance with covariation of cryoloop temperature and vertical, but not lateral, spread of deactivation shows that deactivation of superficial cerebral layers alone was sufficient to completely impair performance on the two motion discrimination tasks, whereas additional deactivation of the deep layers was essential to block performance on the detection and orienting task. Thus, these results show a functional bipartite division of labor between upper and lower cortical layers that is supported by efferent connectional anatomy. Similar bipartite division into upper and lower layers may be a general feature of cerebral cortical architecture, signal processing and guidance of behavior.     

Rewiring of Transcortical Projections to Middle Suprasylvian Cortex Following Early Removal of Cat Areas 17 and 18

Retrograde tracers were injected into middle suprasylvian (MS)cortex of two groups of experimental adult cats that had incurredremoval of visual areas 17 and 18 on either the day of birth(P1), or at 1 month of age (P28). Tracers were also injectedinto the same region of intact and adult ablated control cats.The locations and numbers of labeled neurons in the experimentaland control groups were compared. Following lesions on P1, butat no other age, increased numbers of neurons projected to MScortex. Virtually all of the additional neurons were locatedin the superficial layers of the ventral posterior suprasylvian(vPS) cortex. These results demonstrated that (1) neurons withipsilateral transcortical axons have the potential to reconfiguretheir projections after early, localized cortical damage elsewherein the cortex of the same hemisphere; (2) this reconfigurationinvolves expansion of specific projections and is not a generalizedcapacity of all cortical neurons; (3) the expansion is modalityspecific; and, finally, (4) the ability of cortical neuronsto reorganize projections is limited in time. The expanded projectionfrom vPS to MS cortex may contribute to neuronal compensationsand the sparing of visually guided behaviors previously demonstratedin cats with neonatal visual cortex damage, and is a testamentto the latent capacities immature cerebral cortical neuronspossess to establish new projections following restricted damageto the cerebral cortex early in life.     

Areal organization of the posterior parietal cortex of the ferret (Mustela putorius)

On grounds of electrophysiological mapping, cytoarchitecture, myeloarchitecture and callosal and thalamic connectivity, we have identified two cortical areas in the posterior parietal cortex of the ferret: posterior parietal caudal and rostral (PPc and PPr). These areas occupy the lateral and suprasylvian gyri, from the cingulate sulcus (medially) to the suprasylvian sulcus (laterally) and lie between visual areas 18 and 21 (posteriorly) and the somatosensory areas (anteriorly). Within both areas a coarse representation of the visual field was found and within PPr there was also a representation of the body. Each representation mirrors those within neighboring areas. Cytoarchitectonic and myeloarchitectonic fields within this cortical region did not correspond in any simple way to the physiological representations. The architectonic differences correlate to differential callosal connectivity, with predominant connectivity corresponding to the upper hemifield/head representations. PPr and PPc receive thalamic projections from a different, but overlapping, complement of thalamic nuclei. The superimposition of somatic and visual maps in PPr might relate to the probable role of this area in transforming retinal-centered to body-centered spatial coordinates. The organization of the parietal areas in the ferret resembles that of the flying fox and might unveil a common organizational plan from which the primate posterior parietal cortex evolved.     

Training-induced recovery of visual motion perception after extrastriate cortical damage in the adult cat

Unilateral ibotenic acid lesions of the lateral suprasylvian (LS) cortex severely impair the ability of cats to integrate local motion signals (measured as direction range thresholds) and to extract motion signals from noise (measured as motion signal thresholds) in their contra-lesional visual hemifields. These deficits were found up to several months after the lesions and were limited to thresholds measured with random-dot stimuli, while contrast sensitivity for discriminating the direction of motion of sine-wave gratings remained unaffected. Our goal was to determine whether deficits of complex motion perception could recover and whether the recovery was spontaneous or required retraining. In each cat, a single location in the impaired visual hemifield was selected for visual retraining, which required the animals to discriminate motion direction using random-dot stimuli in which the range of dot directions was varied. Fifteen to 40 days of intensive retraining led to a gradual, complete recovery of motion integration. The recovery was stimulus specific since it did not transfer from direction range to motion signal thresholds, and it was largely restricted to the visual field locations retrained. Delaying the onset of retraining by several days to several months had no significant impact on the extent or rate of recovery. Once recovery was achieved, performance remained stable over a period of several months. These results suggest that recovery of complex visual motion perception after lesions of extrastriate visual cortex is an active process that requires extensive, stimulus- and retinotopically-specific visual retraining.     

Attentional load and sensory competition in human vision: modulation of fMRI responses by load at fixation during task-irrelevant stimulation in the peripheral visual field

Perceptual suppression of distractors may depend on both endogenous and exogenous factors, such as attentional load of the current task and sensory competition among simultaneous stimuli, respectively. We used functional magnetic resonance imaging (fMRI) to compare these two types of attentional effects and examine how they may interact in the human brain. We varied the attentional load of a visual monitoring task performed on a rapid stream at central fixation without altering the central stimuli themselves, while measuring the impact on fMRI responses to task-irrelevant peripheral checkerboards presented either unilaterally or bilaterally. Activations in visual cortex for irrelevant peripheral stimulation decreased with increasing attentional load at fixation. This relative decrease was present even in V1, but became larger for successive visual areas through to V4. Decreases in activation for contralateral peripheral checkerboards due to higher central load were more pronounced within retinotopic cortex corresponding to 'inner' peripheral locations relatively near the central targets than for more eccentric 'outer' locations, demonstrating a predominant suppression of nearby surround rather than strict 'tunnel vision' during higher task load at central fixation. Contralateral activations for peripheral stimulation in one hemifield were reduced by competition with concurrent stimulation in the other hemifield only in inferior parietal cortex, not in retinotopic areas of occipital visual cortex. In addition, central attentional load interacted with competition due to bilateral versus unilateral peripheral stimuli specifically in posterior parietal and fusiform regions. These results reveal that task-dependent attentional load, and interhemifield stimulus-competition, can produce distinct influences on the neural responses to peripheral visual stimuli within the human visual system. These distinct mechanisms in selective visual processing may be integrated within posterior parietal areas, rather than earlier occipital cortex.     

Activation of the human occipital and parietal cortex by pattern and luminance stimuli: neuromagnetic measurements

We compared cortical reactivity to pattern and luminance stimuli by recording evoked responses and spontaneous brain rhythms from 10 subjects with a whole-scalp neuromagnetometer. Hemifield patterns (black-and-white checkerboards) elicited strong contralateral transient activation of the occipital V1/V2 cortex, maximum at 65-75 ms, followed by sustained activation during the 2 s stimulus. Responses to hemifield luminance stimuli also had an occipital component, but they were dominated by activation of the medial parieto-occipital sulcus (POS) 60- 70 ms later. The POS region was equally well activated by foveal and extrafoveal stimuli. The occipital responses to hemifield luminance stimuli differed from those to pattern stimuli in two main aspects: the sustained activation was significantly weaker, and the responses were almost symmetrical, indicating a surprisingly bilateral occipital activation. These effects were similar with foveal and extrafoveal stimuli. The spontaneous 10 Hz alpha rhythm, originating predominantly in the POS region, was suppressed after both stimulus onsets and offsets, more strongly for luminance than pattern stimuli. Activation of the occipital cortex dominated after pattern stimuli, whereas the effect of luminance stimulation was stronger in the parieto-occipital region. The distinct signal distributions in the occipital and POS regions suggest that the two types of stimuli activate the magno- and parvocellular pathways to a varying degree. These findings are also in line with a stronger attention-catching value of the luminance than pattern stimuli.      

Additive effects of emotional content and spatial selective attention on electrocortical facilitation

Affectively arousing visual stimuli have been suggested to automatically attract attentional resources in order to optimize sensory processing. The present study crosses the factors of spatial selective attention and affective content, and examines the relationship between instructed (spatial) and automatic attention to affective stimuli. In addition to response times and error rate, electroencephalographic data from 129 electrodes were recorded during a covert spatial attention task. This task required silent counting of random-dot targets embedded in a 10 Hz flicker of colored pictures presented to both hemifields. Steady-state visual evoked potentials (ssVEPs) were obtained to determine amplitude and phase of electrocortical responses to pictures. An increase of ssVEP amplitude was observed as an additive function of spatial attention and emotional content. Statistical parametric mapping of this effect indicated occipito-temporal and parietal cortex activation contralateral to the attended visual hemifield in ssVEP amplitude modulation. This difference was most pronounced during selection of the left visual hemifield, at right temporal electrodes. In line with this finding, phase information revealed accelerated processing of aversive arousing, compared to affectively neutral pictures. The data suggest that affective stimulus properties modulate the spatiotemporal process along the ventral stream, encompassing amplitude amplification and timing changes of posterior and temporal cortex.     

Cortical dynamics subserving visual apparent motion

Motion can be perceived when static images are successively presented with a spatial shift. This type of motion is an illusion and is termed apparent motion (AM). Here we show, with a voltage sensitive dye applied to the visual cortex of the ferret, that presentation of a sequence of stationary, short duration, stimuli which are perceived to produce AM are, initially, mapped in areas 17 and 18 as separate stationary representations. But time locked to the offset of the 1st stimulus, a sequence of signals are elicited. First, an activation traverses cortical areas 19 and 21 in the direction of AM. Simultaneously, a motion dependent feedback signal from these areas activates neurons between areas 19/21 and areas 17/18. Finally, an activation is recorded, traveling always from the representation of the 1st to the representation of the next or succeeding stimuli. This activation elicits spikes from neurons situated between these stimulus representations in areas 17/18. This sequence forms a physiological mechanism of motion computation which could bind populations of neurons in the visual areas to interpret motion out of stationary stimuli.     

Local cortical function after uncomplicated subdural electrode implantation. Laboratory investigation

OBJECTIVES: Although subdural electrodes are routinely used to map regional brain function, it is unknown if the presence of these implants hinders local cortical function. The authors used psychophysical methods to measure the effect of uncomplicated electrode implantation on local cortical function. METHODS: Local field potentials were used to map receptive fields (RFs) for subdural electrodes that were unilaterally implanted on early visual cortex in 4 patients. After electrode implantation, patients did a task that required them to detect an orientation change in a flashing visual stimulus that was presented either inside the mapped RF or outside the RF in the diametrically opposite portion of the other hemifield. The size of the orientation change was varied to span a wide range of behavioral performance. Psychometric curves were generated by fitting behavioral responses to a logistic function. The threshold was defined as the point at which the fitted function crossed 50% detection. RESULTS: Data were well fit by the logistic function in all 4 patients for both RF and non-RF conditions. None of the volunteers tested showed a statistically significant difference in detection threshold, reaction time, or in the slope of the psychometric function for stimuli presented inside or outside the RF. CONCLUSIONS: Subdural electrodes implanted for extraoperative monitoring do not impair psychophysical performance for a task based on stimuli lying within the RF for recording electrodes. This finding suggests that these electrodes can be used reliably for accurate assessment of regional neurological function.     

Parieto-frontal interactions in visual-object and visual-spatial working memory: evidence from transcranial magnetic stimulation.

This study aimed to investigate whether transcranial magnetic stimulation (TMS) can induce selective working memory (WM) deficits of visual-object versus visual-spatial information in normal humans. Thirty-five healthy subjects performed two computerized visual n-back tasks, in which they were required to memorize spatial locations or abstract patterns. In a first series of experiments, unilateral or bilateral TMS was delivered on posterior parietal and middle temporal regions of both hemispheres after various delays during the WM task. Bilateral temporal TMS increased reaction times (RTs) in the visual-object, whereas bilateral parietal TMS selectively increased RTs in the visual-spatial WM task. These effects were evident at a delay of 300 ms. Response accuracy was not affected by bilateral or unilateral TMS of either cortical region. In a second group of experiments, bilateral TMS was applied over the superior frontal gyrus (SFG) or the dorsolateral prefrontal cortex (DLPFC). TMS of the SFG selectively increased RTs in the visual-spatial WM task, whereas TMS of the DLPFC interfered with both WM tasks, in terms of both accuracy and RTs. These effects were evident when TMS was applied after a delay of 600 ms, but not one of 300 ms. These findings confirm the segregation of WM buffers for object and spatial information in the posterior cortical regions. In the frontal cortex, the DLPFC appears to be necessary for WM computations regardless of the stimulus material.     

Physiological and anatomical evidence for multisensory interactions in auditory cortex

Recent studies, conducted almost exclusively in primates, have shown that several cortical areas usually associated with modality-specific sensory processing are subject to influences from other senses. Here we demonstrate using single-unit recordings and estimates of mutual information that visual stimuli can influence the activity of units in the auditory cortex of anesthetized ferrets. In many cases, these units were also acoustically responsive and frequently transmitted more information in their spike discharge patterns in response to paired visual-auditory stimulation than when either modality was presented by itself. For each stimulus, this information was conveyed by a combination of spike count and spike timing. Even in primary auditory areas (primary auditory cortex [A1] and anterior auditory field [AAF]), approximately 15% of recorded units were found to have nonauditory input. This proportion increased in the higher level fields that lie ventral to A1/AAF and was highest in the anterior ventral field, where nearly 50% of the units were found to be responsive to visual stimuli only and a further quarter to both visual and auditory stimuli. Within each field, the pure-tone response properties of neurons sensitive to visual stimuli did not differ in any systematic way from those of visually unresponsive neurons. Neural tracer injections revealed direct inputs from visual cortex into auditory cortex, indicating a potential source of origin for the visual responses. Primary visual cortex projects sparsely to A1, whereas higher visual areas innervate auditory areas in a field-specific manner. These data indicate that multisensory convergence and integration are features common to all auditory cortical areas but are especially prevalent in higher areas.     

The orbitofrontal cortex and reward

The primate orbitofrontal cortex contains the secondary taste cortex, in which the reward value of taste is represented. It also contains the secondary and tertiary olfactory cortical areas, in which information about the identity and also about the reward value of odors is represented. The orbitofrontal cortex also receives information about the sight of objects and faces from the temporal lobe cortical visual areas, and neurons in it learn and reverse the visual stimulus to which they respond when the association of the visual stimulus with a primary reinforcing stimulus (such as a taste reward) is reversed. However, the orbitofrontal cortex is involved in representing negative reinforcers (punishers) too, such as aversive taste, and in rapid stimulus-reinforcement association learning for both positive and negative primary reinforcers. In complementary neuroimaging studies in humans it is being found that areas of the orbitofrontal cortex (and connected subgenual cingulate cortex) are activated by pleasant touch, by painful touch, by rewarding and aversive taste, and by odor. Damage to the orbitofrontal cortex in humans can impair the learning and reversal of stimulus- reinforcement associations, and thus the correction of behavioral responses when these are no longer appropriate because previous reinforcement contingencies change. This evidence thus shows that the orbitofrontal cortex is involved in decoding and representing some primary reinforcers such as taste and touch; in learning and reversing associations of visual and other stimuli to these primary reinforcers; and in controlling and correcting reward-related and punishment-related behavior, and thus in emotion.     

Differential sensitivity to words and shapes in ventral occipito-temporal cortex

Efficient extraction of shape information is essential for proficient reading but the role of cortical mechanisms of shape analysis in word reading is not well understood. We studied cortical responses to written words while parametrically varying the amount of visual noise applied to the word stimuli. In only a few regions along the ventral surface, cortical responses increased with word visibility. We found consistently increasing responses in bilateral posterior occipito-temporal sulcus (pOTS), at an anatomical location that closely matches the "visual word form area". In other cortical regions, such as V1, responses remained constant regardless of the noise level. We performed 3 additional tests to assess the functional specialization of pOTS responses for written word processing. We asked whether pOTS responses are 1) left lateralized, 2) more sensitive to words than to line drawings or false fonts, and 3) invariant for visual hemifield of words but not other stimuli. We found that left and right pOTS response functions both had highest sensitivity for words, intermediate for line drawings, and lowest for false fonts. Visual hemifield invariance was similar for words and line drawings. These results suggest that left and right pOTS are both involved in shape processing, with enhanced efficiency for processing visual word forms.     

Cortical response field dynamics in cat visual cortex

Little is known about the "inverse" of the receptive field--the region of cortical space whose spatiotemporal pattern of electrical activity is influenced by a given sensory stimulus. We refer to this activated area as the cortical response field, the properties of which remain unexplored. Here, the dynamics of cortical response fields evoked in visual cortex by small, local drifting-oriented gratings were explored using voltage-sensitive dyes. We found that the cortical response field was often characterized by a plateau of activity, beyond the rim of which activity diminished quickly. Plateau rim location was largely independent of stimulus orientation. However, approximately 20 ms following plateau onset, 1-3 peaks emerged on it and were amplified for 25 ms. Spiking was limited to the peak zones, whose location strongly depended on stimulus orientation. Thus, alongside selective amplification of a spatially restricted suprathreshold response, wider activation to just below threshold encompasses all orientation domains within a well-defined retinotopic vicinity of the current stimulus, priming the cortex for processing of subsequent stimuli.     

Integration of local features to a global percept by neural coupling

The integration of different visual attributes into the percept of a single global shape is a central aspect of object processing. In hierarchically organized stimuli with local and global levels, the attentional focus largely determines which level is processed. Here we tested the hypothesis that object processing during attention to the global aspect of the stimulus is characterized by an increased neural coupling between visual areas reflecting the integration of local features. In the present experiment, we used global letters that were constructed by smaller local letters, and a cue signaled which spatial level should be identified. On the local level, only 1 relevant letter was presented laterally in 1 visual hemifield. In contrast, the global letter extended into both hemifields, and the integration of information from both hemispheres was necessary to identify the global stimulus. Therefore, we expected an increased functional coupling between hemispheres during global processing. This hypothesis was investigated using electroencephalographic recordings and an analysis of phase locking and coherence. The results show that stimulus-locked neural coupling within the gamma band (30-40 Hz) across hemispheres in visual cortex increased for global processing after stimulus presentation and could therefore reflect the integration of local visual information.     

Topography of cortical activation differs for fundamental and harmonic frequencies of the steady-state visual-evoked responses. An EEG and PET H215O study

In humans, visual flicker stimuli of graded frequency (2-90 Hz) elicit an electroencephalographic (EEG) steady-state visual-evoked response (SSVER) with the same fundamental frequency as the stimulus and, in addition, a series of harmonic responses. The fundamental component of the SSVER is generated by increased synaptic activity in primary visual cortex (V1). We set out to determine the cortical origin of the harmonic responses in humans. For this purpose, we recorded the SSVERs at 5 different frequencies (5, 10, 15, 25, and 40 Hz) and measured regional cerebral blood flow (rCBF) with positron emission tomography-H(2)(15)O at rest and during visual stimulation at the same frequencies. The rCBF contrast weighted by the amplitude of the SSVERs first harmonics showed activation of a swath of cortex perpendicular to V1, including mostly the inferior half of the parieto-occipital sulcus. This area overlapped minimally with the primary visual cortex activated by the fundamental frequency. A different method, estimating EEG cortical source current density with low-resolution brain electromagnetic tomography, gave the same results. Our finding suggests that the inferior portion of the banks of the parieto-occipital sulci contains association visual cortex involved in the processing of stimuli that can be as simple as a flickering light source.     

Selective attention increases the dependency of cortical responses on visual motion coherence in man

Attention improves visual discrimination and consequently allows to discern stimuli with low signal-to-noise ratios that otherwise would remain undetected. We used magnetoencephalography (MEG) to test whether neuromagnetic responses recorded from occipito-temporal cortex, reflecting the size of visual motion signals embedded in noise (motion coherence), would mirror the perceptual changes induced by attention. Attention directed to a given hemifield increased and decreased the coherence modulation of the MEG response over contralateral and ipsilateral visual cortex, respectively, indicating a change in the neuronal signal-to-noise ratio at the population level.     

Shape discrimination deficits during reversible deactivation of area V4 in the macaque monkey

The role of area V4 in the primate extrastriate cortex has received much attention in recent years. However, the deficit specificity following area V4 ablations has been difficult to determine due to the ablations including area V4 and additional adjacent areas, deficit attenuation and the numerous variations in the results of different research teams. In order to address these issues, we examined the role of area V4 during reversible deactivation of the lower visual field representation within this area while macaque monkeys performed simple pattern discriminations and their eye position was monitored. Specifically, the monkeys were trained to perform a match-to-sample task with the sample stimulus placed within or outside the visual field quadrant represented within the deactivated region of area V4. The sample and match stimuli had the same salience (same size or luminance). Using this approach, we identified significant simple shape discrimination deficits during deactivation of area V4 that did not attenuate with time. Deficits were also identified when the discriminanda were the same figure viewed at different orientations (rotated shapes). In contrast, no deficits were identified during simple hue discriminations. Furthermore, no saccadic eye movement deficits were identified during deactivation of area V4. Therefore, we conclude that deactivation of area V4 yields specific deficits on simple and rotated shape discriminations. These results show that area V4 is an important step in shape and form processing along the ventral visual stream leading to the inferotemporal cortex.     

Visual modulation of neurons in auditory cortex

Our brain integrates the information provided by the different sensory modalities into a coherent percept, and recent studies suggest that this process is not restricted to higher association areas. Here we evaluate the hypothesis that auditory cortical fields are involved in cross-modal processing by probing individual neurons for audiovisual interactions. We find that visual stimuli modulate auditory processing both at the level of field potentials and single-unit activity and already in primary and secondary auditory fields. These interactions strongly depend on a stimulus' efficacy in driving the neurons but occur independently of stimulus category and for naturalistic as well as artificial stimuli. In addition, interactions are sensitive to the relative timing of audiovisual stimuli and are strongest when visual stimuli lead by 20-80 msec. Exploring the underlying mechanisms, we find that enhancement correlates with the resetting of slow (approximately 10 Hz) oscillations to a phase angle of optimal excitability. These results demonstrate that visual stimuli can modulate the firing of neurons in auditory cortex in a manner that depends on stimulus efficacy and timing. These neurons thus meet the criteria for sensory integration and provide the auditory modality with multisensory contextual information about co-occurring environmental events.