Shape-coding in IT cells generalizes over contrast and mirror reversal, but not figure-ground reversal.

We assessed how the visual shape preferences of neurons in the inferior temporal cortex of awake, behaving monkeys generalized across three different stimulus transformations. Stimulus-preferences of particular cells among different polygon displays were correlated across reversed contrast polarity or mirror reversal, but not across figure-ground reversal. This corresponds with psychological findings on human shape judgments. Our results imply that neurons in inferior temporal cortex respond to components of visual shape derived only after figure-ground assignment of contours, not to the contours themselves.

The figure has a shape, but the ground does not: evidence from a priming paradigm.

In four experiments, the authors examined the extent to which the ground interpretation of an edge may receive a shape description. These experiments used the priming effect that shapes have on perceptual judgments on a subsequent trial. A robust reduction in error rates and reaction times was seen when the figural shape was the same as that on the previous trial. This repetition priming effect may be due to activation of the shape description of the figure that remained from the previous trial. In contrast, no priming by the shape of the ground was seen even when the contrast sign of the figure reversed between trials. Priming for figural shapes occurred at a relatively abstract level because it was robust across reversals of contrast and orientation. These data suggest that the figural interpretation of a shape receives a shape description but that the ground does not.

Individual differences in working memory capacity and enumeration.

Two experiments are reported in which subjects performed working memory and enumeration tasks. In the first experiment, subjects scoring low on the working memory task also performed poorly on the attention-demanding "counting" portion of the enumeration task. Yet no span differences were found for the non-attention-demanding "subitizing" portion. In Experiment 2, conjunctive and disjunctive distractors were added to the enumeration task. Although both high and low working memory span subjects were adversely affected by the addition of conjunctive distractors, the effect was much greater for the low-span subjects. Implications from these findings are that differences in working memory capacity correspond to differences in capability for controlled attention.

Visually misguided reaching in Balint's syndrome.

A patient with bilateral parietal damage leading to Balint's Syndrome was tested on his ability to reach to, and to describe the locations of visual targets. RM was better at reaching to targets than he was at describing the locations of the same targets. Moreover, he was better at reaching to targets when he could not see them, compared to when he was reaching with visual guidance. In a final experiment, we found that RM showed strong inhibition of responses to non-target items, even though he had a poor representation of their location in depth. As a result of intact inhibition and impaired depth representation, he ignored both target and non-target items in a given direction. These results suggest that in RM a disturbed visual representation of space disrupts an otherwise relatively intact reaching control system.

Deficits of motor intention following parietal lesions.

Patients with lesions to the right parietal lobe were tested on their ability to reach to targets, or to respond verbally to targets. The targets occurred at the same two spatial locations--to the left and right of the patient--with the task being cued by the color of the target. Patients were able to perform both tasks separately rapidly and without error. However, when the two tasks were interleaved, they had difficulty making a response in the left (contralesional) field when this was different to a response that they had just made. These results suggest that lesions to the parietal cortex may cause a deficit in the coding for motor intention, as well as attention in the contralesional field.

Contrast polarity and face recognition in the human fusiform gyrus.

Functional imaging has revealed face-responsive visual areas in the human fusiform gyrus, but their role in recognizing familiar individuals remains controversial. Face recognition is particularly impaired by reversing contrast polarity of the image, even though this preserves all edges and spatial frequencies. Here, combined influences of familiarity and priming on face processing were examined as contrast polarity was manipulated. Our fMRI results show that bilateral posterior areas in fusiform gyrus responded more strongly for faces with positive than with negative contrast polarity. An anterior, right-lateralized fusiform region is activated when a given face stimulus becomes recognizable as a well-known individual.

Effects of increased response dominance and contextual disintegration on the Stroop interference effect in older adults.

In the study we considered the ability of the relative speed of processing-automaticity (RSOP-A) and contextual disintegration (CD) models of the Stroop interference effect to account for the age-related increase in Stroop interference typically observed in older adults. Findings from the first experiment were partially consistent with predictions of the RSOP-A model because response dominance was greater for older adults than for younger adults. However, the age-related increase in interference was independent of this increase in response dominance, suggesting that factors other than those postulated in the RSOP-A model contributed to the greater interference observed in older adults. Results of the second experiment were consistent with the CD model, which suggests that older adults had difficulty maintaining a color-naming strategy to guide task performance.

The role of attention in a spatial memory task in Alzheimer disease patients.

Memory and attention are interrelated cognitive processes that most likely influence the functioning of each other, yet they are often difficult to distinguish in psychological experiments. Young, aged adults, and patients with Alzheimer disease (AD) were tested on a delay response task measuring spatial memory that also placed high demands on attentional resources. Aged adults performed as well as young, suggesting that neither attentional nor memory abilities were exceeded in either group. However, AD subjects were severely impaired on this task. Two further experiments with AD patients examined the relative contribution of attentional and memory deficits in the performance of this population. Both memory and attentional impairments were found; however, errors due to memory factors were more closely related to severity of disease as measured on the Folstein Mini-Mental State Examination than were errors of attentional origin. These studies demonstrate the necessity of accounting for attentional components in studies examining memory, especially in patients with AD.

Deficit in figure-ground segmentation following closed head injury.

Patient CB showed a severe impairment in figure-ground segmentation following a closed head injury. Unlike normal subjects, CB was unable to parse smaller and brighter parts of stimuli as figure. Moreover, she did not show the normal effect that symmetrical regions are seen as figure, although she was able to make overt judgments of symmetry. Since she was able to attend normally to isolated objects, CB demonstrates a dissociation between figure ground segmentation and subsequent processes of attention. Despite her severe impairment in figure-ground segmentation, CB showed normal 'parallel' single feature visual search. This suggests that figure-ground segmentation is dissociable from 'preattentive' processes such as visual search.

Selective attention in a reaching task: effect of normal aging and Alzheimer's disease.

This study examined the ability of young adults, older adults, and older adults suffering from Alzheimer's disease (AD) to perform a selective reaching task. Normal aging did not increase interference caused by distractors. In contrast, patients with AD showed massively increased effects of distractor interference. AD patients showed a high probability of making responses to distractor items. The proportion of these incorrect responses was related to the inability to use inhibitory processes, which increased with the severity of AD. Responses to distractors occurred despite the fact that patients could discriminate targets and distractors and knew that their responses to distractors were in error. These data suggest that AD patients are impaired in their ability to inhibit incorrect responses.

Externally cued and internally generated selection: differences in distractor analysis and inhibition.

In most studies of selective attention, the experimenter informs the participant what the target is and what action is required. For example, participants may be told to name the red drawing. The present study shows that under such conditions of external selection, distractor inhibition is used to ensure that selection is efficient. In external selection, analysis of distractors is limited, causing later recognition of distractor items to be poor. In contrast, during real-world selection a person may be confronted with a number of potential targets and may have to decide what will be the target for action. Under these conditions of internal selection, inhibition of distracting information does not occur. Moreover, distractors are more fully analyzed and thus better recognized at a later test.

Edge-assignment and figure-ground segmentation in short-term visual matching.

Eight experiments examined the role of edge-assignment in a contour matching task. Subjects judged whether the jagged vertical edge of a probe shape matched the jagged edge that divided two adjoining shapes in an immediately preceding figure-ground display. Segmentation factors biased assignment of this dividing edge toward a figural shape on just one of its sides. Subjects were faster and more accurate at matching when the probe edge had a corresponding assignment. The rapid emergence of this effect provides an on-line analog of the long-term memory advantage for figures over grounds which Rubin (1915/1958) reported. The present on-line advantage was found when figures were defined by relative contrast and size, or by symmetry, and could not be explained solely by the automatic drawing of attention toward the location of the figural region. However, deliberate attention to one region of an otherwise ambiguous figure-ground display did produce the advantage. We propose that one-sided assignment of dividing edges may be obligatory in vision.

Hippocampal lesions cause forgetting in a spatial response task.

UNLABELLED: Monkeys with lesions to the hippocampus and overlying cortex were impaired in making a spatially selective response on the basis of a spatial cue. Their impairment was even more severe on a task in which they were required to make spatial responses on the basis of cues which are not spatially distinct. A second experiment showed that once lesioned monkeys had been trained on a task with spatially distinct stimuli, they were initially able to perform accurately if the aspatial distinctiveness of the cue was reduced. However, their performance declined to chance over four to six trials. These results suggest that lesions to the hippocampus and overlying cortex may cause impairments in memory for the arrangement of visual scenes, including the spatial location of responses. KEYWORDS: spiny neurons, direct pathway, indirect pathways, rat neostriatum

Tilted letters and tilted words: a possible role for principal axes in visual word recognition.

Subjects made lexical decisions to columnar letter strings in which every letter was either upright or tilted 90 degrees clockwise as if the whole letter string had been rotated from the horizontal. Lexical decisions were faster in the latter case. The advantage for the tilted format was also found when all strings were presented in aLtErNaTiNg CaSe or in uppercase, so this advantage cannot be due to preservation of the tilted words' global shape. The cost for the upright-letter format increased with the number of letters in the columnar strings. These data suggest that words recognition may involve shape description or position coding relative to a reference frame based on the principal axis of the letter string.

Afferent connections of the caudolateral orbitofrontal cortex taste area of the primate.

A cortical taste region has recently been identified in the caudolateral orbitofrontal cortex of the macaque. The afferents to this region were investigated by means of retrograde tracing, using six injections of wheatgerm-conjugated horseradish peroxidase. The area of taste cortex was first identified physiologically in all the monkeys used in this anatomical study. The four injections into the middle and posterior part of this region resulted in large numbers of labelled cell bodies in the insular-opercular primary taste cortex. Following the two more anterior injections, label was found predominantly in the caudal part of the cardolateral orbitofrontal cortex itself. None of the injections resulted in labelled cells in the gustatory thalamic nucleus ventralis posterior medialis, pars parvocellularis, although all injections resulted in label of the mediodorsal nucleus of the thalamus. Afferents were also seen from more anterior parts of the orbitofrontal taste cortex, which may represent backprojections from subsequent taste areas. These results suggest that the caudolateral orbitofrontal cortex contains a higher-order taste cortex.

Axis-based neglect of visual shapes.

A new test of "object-centered" visual neglect uses equilateral triangles, with ambiguous principal axes that can be manipulated by context. Three left neglect patients detected gaps in such triangles. The location of the gap relative to the biased principal axis was varied, while maintaining the same egocentric locus. More gaps were missed on the left of the axis. This supports Driver and Halligan's (Cognit. Neuropsychol. 8, 475-496, 1991) claim that neglect can apply to the contralesional side of a shape's principal axis, while avoiding serious flaws in their method. The relation between axis-based neglect and other cases of object-centred neglect is discussed.

Reading of letters and words in a patient with Balint's syndrome.

A patient with bilateral parietal damage, and Balint's syndrome, named visual letters. These were presented individually or within four-letter strings. Solitary letters were identified very accurately. In the case of strings, more letters were correctly reported for words than for nonwords, and more for pronounceable than for unpronounceable nonwords. When required to read words as a whole, performance was better than predicted by letter-reports. These results extend the object-based limitation apparent in Balint's syndrome to the case of reading. The component letters of a string benefit when they form a familiar global object, rather than requiring representation as multiple separate objects. The patient occasionally made homophonic errors when listing the letters in a visual word. This suggests an attempt to bypass visual simultanagnosia by treating the string as a single object, deriving a holistic phonological code for it, and then decomposing this into component letters via spelling rules.

Hippocampal lesions impair spatial response selection in the primate.

This study investigates the role of the primate hippocampus in spatial cognition. Following lesions to the hippocampus, monkeys were unable to learn to direct their responses to different locations in space, based on a spatial cue. This deficit was not due to an impairment in learning to make the responses, since lesioned monkeys were normal when these did not have to be spatially selective. Furthermore, the deficit could not have been due to an impairment in the detection of cues, since lesioned monkeys were normal in their ability to detect visual targets presented over a wide area of space. These results support the view that the hippocampus of the monkey is necessary in order to remember the spatial arrangement of visual scenes in an egocentric manner.

Visual attention and objects: evidence for hierarchical coding of location.

In 5 experiments, it was found that judging the relative location of 2 contours was more difficult when they belonged to 2 objects rather than 1. This was observed even when the 1- and 2-object displays were physically identical, with perceptual set determining how many objects they were seen to contain. Such a 2-object cost is consistent with object-based views of attention and with a hierarchical scheme for position coding, whereby object parts are located relative to the position of their parent object. In further experiments, it was shown that in accord with this hierarchical scheme, the relative location of objects could disrupt judgments of the relative location of object parts, but the reverse did not occur. This was found even when the relative position of the parts could be judged more quickly than that of the objects.

Visual extinction and stimulus repetition.

Abstract Five patients with visual extinction following unilateral brain injury were briefly presented with colored letters in either or both visual fields, and required to report and locate the colors or the shapes. On double simultaneous stimulation, they tended to miss the event contralateral to their lesion. This extinction was increased when the two stimuli were the same on the reported dimension, Similarity on the irrelevant dimension had no effect. These data suggest that extinguished colors and shapes may be correctly extracted by the visual system (when task-relaant) even though they are unavailable for verbal report. An analogy is made with the phenomena of "repetition blindness" in normal observers, and it is proposed that extinction may reflect failure in a token-individuation process for correctly extracted visual types.