Contribution of neurons in monkey parietal cortex to a visual grouping

Introduction 　Visual grouping is an essential component of the visual perception of objects. It is the process by which multiple discrete elements are bound into a single object. For example, if multiple dots with the same color are arranged along a straight line, these dots are grouped together and recognized as a single linear object. This grouping is caused by bottom-up factors such as similarity and continuity of the dots. It is known that visual grouping is also affected by top-down factors such as prior knowledge and past experience with the objects. Neurological observations made in human patients and in fMRI studies of healthy human subjects suggest that the posterior parietal cortex plays a key role in visual grouping. It remains unknown, however, how parietal cortex are involved in visual grouping. Relationship between neural responses and visual grouping in the monkey parietal cortex 　To investigate the neuronal mechanisms underlying visual grouping, we designed a grouping detection task controlled by top-down attention, and performed extra-cellular single unit recording from lateral bank of intra-parietal sulcus (L-IPS) while the task was being performed by monkeys. The visual stimuli consisted of multiple discrete dots, and the monkeys were required to detect the target defined by specific arrangements of the dots. In addition, we manipulated the monkeys’ attention to the grouping of the elements, and examined the effect of attention on the neuronal responses. The visual stimuli were composed of 5 square black or white dots (1.2 deg at the edge) arranged in a cross. A total of 20 types of visual stimuli composed of different arrangements of dots were prepared. In four of the 20 stimuli, three dots with the same contrast (either black or white) were aligned either horizontally or vertically and served as the target. The remaining 16 stimuli were non-targets. The target stimuli were characterized by two visual features: the orientation of the three dots with the same contrast that was either horizontal or vertical (target orientation) and the contrast of three-aligned dots, which was either white or black (target contrast). Visual stimuli were presented multiple times in one trial. The monkeys made behavioral response via a lever, and had to release the lever within 600 ms after the onset of the target to obtain a liquid reward. The monkeys performed the detection task while their attention was directed towards a particular orientation. The attended orientation was controlled either by a visual cue or by a biased block design. 　We recorded the activities of 107 single neurons in the L-IPS while two monkeys performed a grouping detection task. We found that L-IPS neurons selectively responded to the visual stimulus, and a majority of neurons exhibited stronger selectivity for the target orientation than the target contrast. This orientation selectivity was enhanced when the target orientation matched the attended orientation. Moreover, the orientation-selective responses correlated with the monkeys' behavior. These results suggest that L-IPS neurons play important roles in the visual grouping and detection of objects comprised of discrete elements. Activities of different cell classes in visual grouping 　Although it is known that there are two functional classes of cortical neurons, excitatory pyramidal neurons and inhibitory interneurons, it remains largely unknown how these two classes contribute to visual perception and cognition. Recently, several attempts have been made to classify extracellularly recorded neurons according to known differences in the waveforms of their action potentials (e.g., Mitchell et al., 2007, Neuron). These studies suggest that classification of neuron type will provide valuable new information that could be crucial to understanding neural processing within local circuits in the cerebral cortex. 　In order to examine how different classes of neurons are involved in visual grouping, we classified recorded neurons according to the waveforms of their action potentials, and compared the response properties of classified neurons. 　We found that putative pyramidal neurons, which had broader action potentials, exhibited selectivity for the target orientation, and the selectivity was enhanced by attention. By contrast, putative inhibitory neurons, which had narrower action potentials, did not exhibit such selectivity or enhancement. Instead interneurons responded more strongly to the target stimuli than to the non-targets, regardless of the orientation of the target. These results suggest that different classes of parietal neurons contribute differently to the visual grouping of discrete elements. Feedback projection linking the visual fields surrounding the blind spot 　Classification of L-IPS neurons showed that pyramidal neurons exhibited selectivity for the target stimulus, and clearly indicates that L-IPS neurons signal inform ...

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