Effects of figural complexity and target-distracter similarity on decay rate in short-term visual memory

We examined the effects of figural complexity and target-distracter similarity on decay rate in short-term visual memory (STVM) for closed contours. The subject's task was to compare two stimuli presented at different points in time. Experiment 1 showed that even a simple pattern with 6 convex parts cannot be handled in STVM for several seconds without loss. Experiments 1 and 2 indicated that decay rate tends to be higher for patterns with higher complexity and for patterns with higher target-distracter similarity regardless of pattern complexity. These results are interpreted that the value of each convex part represented in STVM is an independent Gaussian random variable and that memory noise increases with retention intervals for each convex part separately. Furthermore, they suggest that memory for local features in STVM decays faster. Yet visual rehearsal appears to operate effectively on memorizing even complex patterns. It is concluded that visually presented patterns can be retained in STVM with relatively high fidelity for prolonged periods of time, although some loss of precision occurs inevitably.     

Effects of altered visual input upon the development of the visual and somatosensory representations in the hamster's superior colliculus

The right superior colliculus and right eye were ablated in hamsters within 12 h of birth and the visual and somatosensory representations in the remaining (left) superior colliculus were evaluated using standard single unit recording and receptive field mapping techniques when the animals reached adulthood (at least 3 months of age). In a number of the hamsters used for recording, injections of [3H]leucine were made into the left eye 6-10 days prior to the terminal experiment. This was done to insure that the neonatal lesions did, in fact, produce the extensive recrossing of retinal fibers demonstrated by others who have employed this preparation. All of the hamsters which received [3H]leucine injections prior to the recording experiment exhibited a markedly expanded ipsilateral retinocollicular projection and retinal axons which recrossed the midline at the level of the tectum. The recording experiments showed further that this projection resulted in a visual map which was generally mirror symmetric to that in normal hamsters. There were, however, numerous irregularities and discontinuities in this representation and, in a few hamsters, it appeared almost completely disorganized. There were also a number of abnormalities in the somatosensory representation in the deep tectal laminae of the neonatally brain damaged hamsters. There was a substantial increase in the number of cells with receptive fields that extended onto the ipsilateral side of the body, neurons with split receptive fields were recorded and there were changes in the magnification of different portions of the body surface. These alterations did not, however, change the organization of the somatosensory map in a manner which brought it into alignment with the visual representation in the superficial laminae. Nevertheless, additional recording experiments in animals subjected to enucleation of both eyes and ablation of the superficial laminae of one superior colliculus did indicate that the existence of the aberrant retinal projection was a necessary condition for the somatosensory abnormalities which we observed. Additional anterograde and retrograde tracing experiments demonstrated only one abnormality in the organization of the somatosensory afferent input to the remaining colliculus. In 75% of the brain damaged hamsters, there was a weak crossed projection from the sensorimotor cortex that was never observed in normal animals. Ablation of this cortex at the time of the recording experiment did not, however, reduce the incidence of abnormal somatosensory receptive fields in these hamsters.     

Modulation of similarity on the distraction resistance of visual working memory representation

Controversy exists regarding the distraction resistance of priority items in visual working memory (VWM). The protection account proposes that high-priority items resist distraction more than low-priority items, while the vulnerability account proposes that distractors impair high-priority items more. We proposed another available resource threshold account: distraction will not impair items if available resources exceed their threshold needed for sufficient processing. Using a change-detection paradigm, we manipulated item priority by color similarity and inserted distractors during VWM retention. We investigated the effect of similarity on distraction resistance of relevant (color) and irrelevant (shape) feature representations (Experiments 1–2), and the neural mechanism of this effect using event-related potentials (ERPs; Experiment 3). Behavioral results showed distractors impaired the accuracy of dissimilar items when relevant features were memorized and of similar items when irrelevant features were memorized under simultaneous presentation of similar and dissimilar items. Moreover, distractors impaired the accuracy of dissimilar items when relevant features were memorized and of both similar and dissimilar items when irrelevant features were memorized under separate presentation of items. ERP results showed a smaller negative slow wave and P2 but larger N2 under the similar condition. Similarity protected relevant features of similar items against distraction by reducing memory load, decreasing attentional resources allocated to distractors, and strengthening inhibition of distractors. However, similarity did not protect irrelevant features of similar items. Our results support the available resource threshold account, suggesting that VWM is a flexible and intelligent system despite its limited capacity.     

An electrophysiological measure of access to representations in visual working memory

Previous research has demonstrated that the maintenance of visual information in working memory is associated with a sustained posterior contralateral negativity. Here we show that this component is also elicited during the spatially selective access to visual working memory. Participants memorized a bilateral visual search array that contained two potential targets on the left and right side. The task-relevant side was signalled by post-cues that were presented either 150 ms after array offset or after a longer interval (700–1000 ms). Enhanced negativities at posterior electrodes contralateral to the cued side of a target were elicited in response to both early and late post-cues, suggesting that they reflect not only memory maintenance, but also processes involved in the access to stored visual working memory representations. Results provide new electrophysiological evidence for the retinotopic organization of visual working memory.     

Guidance of visual attention from working memory contents depends on stimulus attributes

Recent work shows that items maintained in working memory could guide the orienting of attention in visual search, demonstrating an interesting interaction between working memory and attention. For such guidance effect, some studies emphasize the importance of search task type: for fixed-target search, task-irrelevant memory items stay inside the attentional focus and guides attention; for varied-target search, these items stay outside attentional focus and do not guide attention. With two experiments, we showed that stimulus attribute of items held in working memory can play an important role in attentional guidance. The first behavioral experiment duplicated a previous fixed-target search paradigm demonstrating robust guidance effect but failed to find such effect when the stimuli was simply changed from colored shapes to complex artificial shapes. The second event-related potential experiment duplicated a previous varied-target search paradigm that used complex shapes but did not observe any guidance effect. A clear guidance effect was found when we changed the stimuli to colored shapes. The results suggest that attentional guidance from working memory depends on stimulus attributes of the items held in working memory. When effective attribute is used, task-irrelevant items that stay outside the attentional focus are still able to guide attention.     

The role of stimulus type in age-related changes of visual working memory

Aging is accompanied by increasing difficulty in working memory associated with the temporary storage and processing of goal-relevant information. Face recognition plays a preponderant role in human behavior, and one might therefore suggest that working memory for faces is spared from age-related decline compared to socially less important visual stimulus material. To test this hypothesis, we performed working memory (n-back) tasks with two different visual stimulus types, namely faces and doors, and compared them to tasks with primarily verbal material, namely letters. Age-related reaction time slowing was comparable for all three stimulus types, supporting hypotheses on general cognitive and motor slowing. In contrast, performance substantially declined with age for faces and doors, but little for letters. Working memory for faces resulted in significantly better performance than that for doors and was more sensitive to on-line manipulation errors such as the temporal order. All together, our results show that even though face perception might play a specific role in visual processing, visual working memory for faces undergoes the same age-related decline as it does for socially less relevant visual material. Moreover, these results suggest that working memory decline cannot be solely explained by increasing vulnerability in prefrontal cortex related to executive functioning, but indicate an age-related decrease in a visual short-term buffer, possibly located in the temporal cortex.     

On the structure of visual similarity (author's transl)

The purpose of the present study is to investigate the structure of visual similarity. Stimuli were two series of geometric figures, one of them was 16 rectangles (series A) and the other 18 "apparent" cubes (series B). Each stimulus series was given to eleven adults and eleven infants (five years old). Kruskal's nonmetric MDS was applied to observed (subjects mean average) similarity data. The main results were as follows: (a) The Euclidean model was more suited to the data than the city block model. (b) The number of necessary dimensions was two (series A), or three (series B) in both groups of the subjects.     

Effects of aging on visual recognition memory in the rhesus monkey

As part of an effort to develop a primate model of human age-related memory dysfunction, performance by six rhesus monkeys 26 to 27 years of age was compared to that of six young adult monkeys (four to five years of age) on a trial unique delayed nonmatching to sample (DNMS) task. This task assesses the monkey's ability to identify a novel from a familiar stimulus over a delay and resembles closely clinical tests that are used to assess memory function in geriatric patients. The task was presented in three stages: acquisition, delays and lists. As a group, aged monkeys were impaired relative to the young adult group on all three conditions. However, within the aged group, individual cases of efficient performance were observed. Error analyses of item positions of the lists condition revealed the absence of enhanced performance for items presented at the end of a list by aged animals, suggesting an abnormal sensitivity to proactive interference. The finding of a recognition impairment with age is in parallel with studies of normal human aging and lends support to the notion that the rhesus monkey is a suitable animal model of human aging.     

Effects of non-focal brain dysfunctioin on visual memory

Investigated the effects of non-focal brain damage on visual memory abilities with a sample of 114 Ss with confirmed brain-dysfunction and 71 controls who completed the Rey-Osterrieth Complex Figure Test. In contrast to the findings of earlier studies of patients with focal brain-damage. right-hemisphere lesioned Ss could not be discriminated from left-hemisphere lesioned Ss on the basis of impaired visual recall. All three brain-damaged groups, however, performed more poorly than the control group in both the copy and recall phases of the task. Age and intelligence also were found to be related inversely to task performance. Implications of these data for clinical neuropsychological assessment are discussed.     

Effects of similarity and history on neural mechanisms of visual selection.

To investigate how the brain combines knowledge with visual processing to locate eye movement targets, we trained monkeys to search for a target defined by a conjunction of color and shape. On successful trials, neurons in the frontal eye field not only discriminated the target from distractors, but also discriminated distractors that shared a target feature as well as distractors that had been the search target during the previous session. Likewise, occasional errant saccades tended to direct gaze to distractors that either resembled the current target or had been the previous target. These findings show that the frontal eye field is involved in visual and not just motor selection and that visual selection is influenced by long-term priming. The data support the hypothesis that visual selection can be accomplished by parallel processing of objects based on their elementary features.     

Decoding task and stimulus representations in face-responsive cortex

Observers can deliberately attend to some aspects of a face (e.g. emotional expression) while ignoring others. How do internal goals influence representational geometry in face-responsive cortex? Participants watched videos of naturalistic dynamic faces during MRI scanning. We measured multivariate neural response patterns while participants formed an intention to attend to a facial aspect (age, or emotional valence), and then attended to that aspect, and responses to the face's emotional valence, independent of attention. Distinct patterns of response to the two tasks were found while forming the intention, in left fronto-lateral but not face-responsive regions, and while attending to the face, in almost all face-responsive regions. Emotional valence was represented in right posterior superior temporal sulcus and medial prefrontal cortex, but could not be decoded when unattended. Shifting the focus of attention thus alters cortical representation of social information, probably reflecting neural flexibility to optimally integrate goals and perceptual input.     

Effects of stimulus orientation on spatial frequency function of the visual evoked potential

Visual performance is better in response to vertical and horizontal stimuli than oblique ones in many visual tasks; this is called the orientation effect. In order to elucidate the electrophysiological basis of this psychophysical effect, we studied the effects of stimulus orientation on the amplitudes and latencies of visual evoked potentials (VEPs) over different spatial frequencies of the visual stimulation. VEPs to sinusoidal gratings at four orientations (vertical, horizontal, and oblique at 45 degrees and 135 degrees) with eight spatial frequencies (0.5-10.7 cycles/deg) at reversal rates of 1 Hz and 4 Hz were recorded in nine subjects. At 1-Hz stimulation, the amplitude and latency of P100 were measured. At 4-Hz stimulation, VEPs were Fourier-analyzed to obtain phase and amplitude of the second harmonic response (2F). At 1-Hz stimulation, P100 latencies were decreased for oblique stimuli compared with those for horizontal and vertical stimuli at lower spatial frequencies. Conversely, those for oblique stimuli were increased compared with those for horizontal and vertical stimuli at higher spatial frequencies. At 4-Hz stimulation, spatial tuning observed in 2F amplitude of the oblique gratings shifted to lower spatial frequencies when compared with those of vertical stimulation. The alteration of the VEP spatial frequency function caused by the oblique stimuli was in good agreement with the orientation effect observed in psychophysical studies. Our study may have a clinical implication in that VEP testing with stimuli in more than one orientation at slow and fast temporal modulations can be useful in evaluating neurological disease affecting the visual system.     

A colour-dependent abnormality in human visual detection of stimulus motion and spatial structure

We have studied the wavelength dependence of visual responses in a single human subject, M.W., who has an unusual visual defect. It is shown that for red stimuli, both spatial resolution for grating patterns and detection of target movement are grossly abnormal. On the basis of these and other [8] experimental data, we argue that M.W.'s response pattern implies interdependent central visual processing of colour, movement and spatial structure. We examine this conclusion in relation to recent anatomical and electrophysiological findings.     

Interhemispheric asymmetries in the visual evoked response: Effects of stimulus lateralisation and task

VER's were elicited by briefly presented lateralised letters in two experiments. In experiment 1 the subjects engaged in verbal processing of the letters and in experiment 2 they engaged in visuospatial processing. In both experiments the latencies of the first two components of the VER's were consistent with previous findings and with the anatomy of the retino-cortical visual pathways.The pattern of hemisphere asymmetries of the amplitudes of the middle and late components differed in the two experiments in that (i) in the second experiment the amplitude of the middle component from the left hemisphere did not vary with field of stimulus presentation although this was the case in the first experiment and (ii) a hemisphere asymmetry (left greater than right) was observed in the late component only in the second experiment.The results are interpreted as evidence of differences in the cerebral processing of the stimuli in respect of the task demands of the two experiments.     

A dissociation in infants' memory for stimulus size: evidence for the early development of multiple memory systems

Adults' memory performance on recognition (explicit memory) tests is sensitive to stimulus size, but their performance on priming (implicit memory) tests is not. This memory dissociation is taken as evidence for two, functionally distinct memory systems. Young infants, however, are thought to possess only a single representational system that supports implicit memory; the system that supports explicit memory is thought not to mature before 8-9 months of age. In two experiments with 54 infants, we asked if 3-month-olds exhibit a memory dissociation for stimulus size on recognition and priming tests. All infants learned to move a mobile displaying +s of a given size. In Experiment 1, infants recognized +s in the original size but not 33% smaller or larger. In Experiment 2, +s were effective memory primes in a reactivation task, irrespective of size. The finding that young infants exhibit a memory dissociation for stimulus size adds to growing evidence that two memory systems are functional from early in development.     

The effect of visual experience on the development of stimulus specificity by kitten cortical neurones

1. 284 single cortical neurones were studied in area seventeen of twenty-five normal kittens and of fifteen kittens, binocularly deprived, whose first visual experience had been delayed until the experiment by bilateral lid-suture. Both normal and binocularly deprived kittens ranged in age from 1 to 6 weeks.2. The optimal, binocularly presented, visual stimulus and receptive fields were determined for each neurone by varying target configuration, speed and direction of movement and the prism-induced alignment of both eyes. Repetitive, controlled stimulation in eighty-four cases allowed quantitative estimates to be made of the response selectivity for the target configuration (spot vs. line), the direction of target motion and the prism-induced disparity between the retinal images of the binocular target.3. Before the fourth post-natal week neurones from both normal and binocularly deprived cortex showed similar properties: selectivity for direction of target motion was present in both preparations but both lacked binocular specificity and dependence on target configuration.4. After the fourth week, normal kittens had increasing numbers of neurones with selective responses which were dependent upon target configuration and the degree of binocular misalignment. The proportion of selective neurones approached the adult value after the fifth week.5. The cortex of binocularly deprived kittens failed to show an increase of selectivity with age, and of 150 neurones, sixty-two were visually unresponsive, two showed selectivity which was dependent upon target configuration and none showed selectivity for prism-induced retinal disparity.6. The data are not consistent with the hypothesis that the highly specific response properties of visual cortical neurones can develop without appropriate visual experience. Innate mechanisms appear to be sufficient for the development of the excitatory connexions producing motion sensitivity and receptive field location on both retinas, but patterned visual experience is necessary for the ;fine-tuning' which vetoes responses to stimuli with non-optimal configuration or binocular disparity.     

Effects of attention and stimulus interaction on visual responses of inferior temporal neurons in macaque

1. Extracellular discharges were recorded from neurons in the inferior temporal cortex (area TE) of three macaque monkeys while they performed visual fixation and pattern discrimination tasks. For the pattern discrimination task, monkey was trained to release the lever quickly at the onset of one of two pattern stimuli and to release the lever at the dimming of the other pattern. During this task, neutral light stimulus (light bar) to which the monkey was not required to respond was presented once a trial either prior to the onset of the discriminandum or during presentation of the pattern that dimmed later. The neuronal activities evoked by the neutral stimulus under these two conditions were compared. 2. When the discriminanda were located at the center or at 5 degrees in the contralateral visual field, one-half of the neurons showed significantly smaller responses to the neutral stimulus when it was presented during presentation of the dimming pattern than when it was presented prior to the onset of the discriminandum. 3. The suppressive effect depended on the location of the two stimuli. When the neutral stimulus was located in the ipsilateral visual field and the pattern was located in the contralateral visual field, the response to the neutral stimulus was suppressed. However, when the pattern was located in the ipsilateral visual field (5 degrees visual angle), still within the receptive field for many neurons, the suppressive effect of the pattern on the response to the neutral stimulus in the contralateral visual field was almost undetectable. 4. When the pattern was located nearer the fovea than was the neutral stimulus, the suppressive effect was greater than when the pattern was located more peripherally to the neutral stimulus. Different from the receptive field of more primary visual neurons, this suppressive effect did not appear to be related to the neuron's responsiveness to the patterns nor to precise stimulus location in the receptive field. 5. The magnitude of suppression by the attended pattern on the visual response during the pattern discrimination task correlated with the suppression noted in the presence of a fixation spot during the fixation tasks, while the animals did not fixate on the attended pattern. The response of some neurons to the neutral stimulus prior to pattern presentation during the pattern discrimination task was enhanced slightly compared with the response recorded during the simple fixation task.(ABSTRACT TRUNCATED AT 400 WORDS)     

Stimulus- and response-locked neuronal generator patterns of auditory and visual word recognition memory in schizophrenia

Examining visual word recognition memory (WRM) with nose-referenced EEGs, we reported a preserved ERP ‘old-new effect’ (enhanced parietal positivity 300–800 ms to correctly-recognized repeated items) in schizophrenia ([Kayser, J., Bruder, G.E., Friedman, D., Tenke, C.E., Amador, X.F., Clark, S.C., Malaspina, D., Gorman, J.M., 1999. Brain event-related potentials (ERPs) in schizophrenia during a word recognition memory task. Int. J. Psychophysiol. 34(3), 249–265.]). However, patients showed reduced early negative potentials (N1, N2) and poorer WRM. Because group differences in neuronal generator patterns (i.e., sink-source orientation) may be masked by choice of EEG recording reference, the current study combined surface Laplacians and principal components analysis (PCA) to clarify ERP component topography and polarity and to disentangle stimulus- and response-related contributions. To investigate the impact of stimulus modality, 31-channel ERPs were recorded from 20 schizophrenic patients (15 male) and 20 age-, gender-, and handedness-matched healthy adults during parallel visual and auditory continuous WRM tasks. Stimulus- and response-locked reference-free current source densities (spherical splines) were submitted to unrestricted Varimax-PCA to identify and measure neuronal generator patterns underlying ERPs. Poorer (78.2 ± 18.7% vs. 87.8 ± 11.3% correct) and slower (958 ± 226 vs. 773 ± 206 ms) performance in patients was accompanied by reduced stimulus-related left-parietal P3 sources (150 ms pre-response) and vertex N2 sinks (both overall and old/new effects) but modality-specific N1 sinks were not significantly reduced. A distinct mid-frontal sink 50-ms post-response was markedly attenuated in patients. Reductions were more robust for auditory stimuli. However, patients showed increased lateral-frontotemporal sinks (T7 maximum) concurrent with auditory P3 sources. Electrophysiologic correlates of WRM deficits in schizophrenia suggest functional impairments of posterior cortex (stimulus representation) and anterior cingulate (stimulus categorization, response monitoring), primarily affecting memory for spoken words.