Does parietal cortex contribute to feature binding?

We studied the involvement of the right parietal cortex in visual conjunction search, where two features are present in the array and spatial attention and feature binding is required, and in subset search, where two features are also present but only one of them is needed in order to group stimuli together (the subset) and allow parallel processing without the need for feature binding. Six patients with right parietal lobe lesions, six age-matched controls, and three control patients with left parietal lesions were tested on these two tasks. Patients with right parietal lesions were significantly slower than normal controls in the conjunction task, especially for target-absent trials. In the subset condition, neither normal control subjects nor patients with left parietal damage showed a difference between target-present and target-absent trials whereas right parietal patients showed a significant difference between target-present and target-absent responses. The results suggest a role for the right parietal cortex in shifting attention to the next stimulus once binding of features has taken place or selecting spatial areas containing the desired feature in a subset search, but that parietal cortex is not required for binding the features of the object.

Temporal aspects of visual search studied by transcranial magnetic stimulation.

Transcranial magnetic stimulation was applied over the parietal visual cortex of subjects while they were performing 'pop-out' or conjunction visual search tasks in arrays containing eight distractors. Magnetic stimulation had no detrimental effect on the performance of pop-out search, but did significantly increase reaction times on conjunction search when stimulation was applied over the right parietal cortex 100 msec after the onset of the visual display for trials when the target was present. Target absent reaction times were elevated when stimulation was applied 160 msec after array onset. Stimulation had no effect on the number of errors made. The results suggest that a sub-region of the right parietal lobe is important for conjunction search but not for pre-attentive pop-out. The result from target present trials is consistent with timing data from studies of single cells in monkeys and the hypothesis that parietal areas generate a signal that projects back to extrastriate visual areas to enhance the processing of features in a restricted part of the visual field. The timing of the effect indicates that transcranial stimulation disrupts the mechanisms underlying the focal attention necessary for feature binding in conjunction search. The effects of TMS on target absent trials are interpreted in terms of fronto-parietal connections and the role of frontal cortex in decision-making. The results also highlight the efficacy of transcranial magnetic stimulation as a complement to other spatial and temporal imaging techniques.

Cortical plasticity in perceptual learning demonstrated by transcranial magnetic stimulation.

Performance on a wide range of perceptual tasks improves with practice. Most accounts of perceptual learning are concerned with changes in neuronal sensitivity or changes in the way a stimulus is represented. Another possibility is that different areas of the brain are involved in performing a task during and after learning it. Here, we demonstrate that the right parietal cortex is involved in novel but not learned visual conjunction search. We observed that single pulse transcranial magnetic stimulation (TMS) to the right parietal cortex impairs visual conjunction search when the stimuli are novel and require a serial search strategy, but not once the particular search task has been learned. The effect of TMS returns when a different, novel, serial search task is presented.

Cortical plasticity in perceptual learning demonstrated by transcranial magnetic stimulation.

Performance on a wide range of perceptual tasks improves with practice. Most accounts of perceptual learning are concerned with changes in neuronal sensitivity or changes in the way a stimulus is represented. Another possibility is that different areas of the brain are involved in performing a task while learning it and after learning it. Here we demonstrate that the right parietal cortex is involved in novel but not learned visual conjunction search. We observed that single pulse transcranial magnetic stimulation (TMS) to the right parietal cortex impairs visual conjunction search when the stimuli are novel and require a serial search strategy, but not once the particular search task has been learned. The effect of TMS returns when a different, novel, serial search task is presented.

The role of the parietal cortex in visual attention--hemispheric asymmetries and the effects of learning: a magnetic stimulation study.

Our previous studies of the role of the parietal cortex in visual learning and attention showed that the right parietal cortex is required for normal performance on conjunction visual search tasks but that its role depends on whether subjects are naive or trained on the task. Here we extend these findings in two Experiments. Experiment 1 shows that magnetic stimulation of the left parietal cortex also impairs performance (measured as reaction time) on conjunction visual search tasks, but only when the target is present in the right (contralateral) visual field. Stimulation of the same region on a feature detection task speeds up performance significantly when the target is in the left (ipsilateral) visual field. Experiment 2 explores further the role of the right parietal cortex in learning conjunction search tasks. Stimulation of the right parietal cortex in subjects who had already trained on some visual search tasks did not impair performance on a novel motion/form conjunction task even though the search was clearly serial. Stimulation of area V5, however, severely disrupted performance on the same task. These data indicate that the role of the parietal cortex may change much earlier in the course of training than initially thought.

Evidence accumulation in cell populations responsive to faces: an account of generalisation of recognition without mental transformations.

In this paper we analyse the time course of neuronal activity in temporal cortex to the sight of the head and body. Previous studies have already demonstrated the impact of view, orientation and part occlusion on individual cells. We consider the cells as a population providing evidence in the form of neuronal activity for perceptual decisions related to recognition. The time course on neural responses to stimuli provides an explanation of the variation in speed of recognition across different viewing circumstances that is seen in behavioural experiments. A simple unifying explanation of the behavioural effects is that the speed of recognition of an object depends on the rate of accumulation of activity from neurones selective for the object, evoked by a particular viewing circumstance. This in turn depends on the extent that the object has been seen previously under the particular circumstance. For any familiar object, more cells will be tuned to the configuration of the object's features present in the view or views most frequently experienced. Therefore, activity amongst the population of cells selective for the object's appearance will accumulate more slowly when the object is seen in an unusual view, orientation or size. This accounts for the increased time to recognise rotated views without the need to postulate 'mental rotation' or 'transformations' of novel views to align with neural representations of familiar views.

Effect of image orientation and size on object recognition: responses of single units in the macaque monkey temporal cortex.

This study examined how cells in the temporal cortex code orientation and size of a complex object. The study focused on cells selectively responsive to the sight of the head and body but unresponsive to control stimuli. The majority of cells tested (19/26, 73%) were selectively responsive to a particular orientation in the picture plane of the static whole body stimulus, 7/26 cells showed generalisation responding to all orientations (three cells with orientation tuning superimposed on a generalised response). Of all cells sensitive to orientation, the majority (15/22, 68%) were tuned to the upright image. The majority of cells tested (81%, 13/16) were selective for stimulus size. The remaining cells (3/16) showed generalisation across four-fold decrease in size from life-sized. All size-sensitive cells were tuned to life-sized stimuli with decreasing responses to stimuli reduced from life-size. These results do not support previous suggestions that cells responsive to the head and body are selective to view but generalise across orientation and size. Here, extensive selectivity for size and orientation is reported. It is suggested that object orientation and size-specific responses might be pooled to obtain cell responses that generalise across size and orientation. The results suggest that experience affects neuronal coding of objects in that cells become tuned to views, orientation, and image sizes that are commonly experienced. Models of object recognition are discussed.