Receptive Field Characteristics That Allow Parietal Lobe Neurons to Encode Spatial Properties of Visual Input: A Computational Analysis

A subset of visually sensitive neurons in the parietal lobe apparently can encode the locations of stimuli, whereas visually sensitive neurons in the inferotemporal cortex (area IT) cannot. This finding is puzzling because both sorts of neurons have large receptive fields, and yet location can be encoded in one case, but not in the other. The experiments reported here investigated the hypothesis that a crucial difference between the IT and parietal neurons is the spatial distribution of their response profiles. In particular, IT neurons typically respond maximally when stimuli are presented at the fovea, whereas parietal neurons do not. We found that a parallel-distributed-processing network could map a point in an array to a coordinate representation more easily when a greater proportion of its input units had response peaks off the center of the input array. Furthermore, this result did not depend on potentially implausible assumptions about the regularity of the overlap in receptive fields or the homogeneity of the response profiles of different units. Finally, the internal representations formed within the network had receptive fields resembling those found in area 7a of the parietal lobe.Psycholog

Effector-independent And Effector-dependent Sequence Representations Underlie General And Specific Perceptuomotor Sequence Learning

Perceptuomotor sequence learning could be due to learning of effector-independent sequence information (e.g., response locations), effector-dependent information (e.g., motor movements of a particular effector), or both. Evidence also suggests that learning of statistical regularities in sequences (generalregularity learning) and specific sequences (specific-sequence learning) are dissociable. The authors examined the degree to which general and specific-sequence learning rely on effector-independent and effector-dependent representations. During training, participants typed sequences that followed a construction rule with a subset of sequences repeatedly processed. At test, effector-independent and effector-dependent learning was examined with respect to generalregularity and specific-sequence learning. Results suggest that general-regularity learning is subserved by effector-independent sequence representations, whereas specific-sequence learning is subserved by effector-dependent sequence representations, further dissociating these types of learning

Form-specific visual priming for new associations in the right cerebral hemisphere.

The ability to distinguish specific instances in the same abstract category of visual form is an important human faculty. One can differentiate a particular cup from other cups as well as an individual's signature from other written versions of the same set of letters. An interesting aspect of this ability is that most theories of visual-form recognition, aside from theories that address only face recognition (e.g., VISUAL-FORM SUBSYSTEMS We hypothesize that a specific visual-form (SVF) subsystem underlies recognition of specific instances of forms and operates more effectively in the right cerebral hemisphere (RH) than in the left cerebral hemisphere (LH). In contrast, an abstract visual-form (AVF) subsystem supports recognition of abstract categories of forms and operates more effectively in the LH than in the RH. These subsystems likely focus on different properties of visual-form inputs to achieve different goals. SVF Subsystem In order to produce different outputs when different instances in the same abstract category appear as inputs, an SVF subsystem must process a form's visually distinctive information effectively. Visually distinctive information refers to the specific structural properties that vary across the different instances in an abstract category. For example, the lowercase form "p" and the uppercase form "P" have structurally distinct vertices at both connecting points between the vertical line and the enclosed space, 539 Copyright 1996 Psychonomic Society, Inc. In three experiments, we examined the internal processing mechanisms of relatively independent visual-form subsystems. Participants first viewed centrally presented word pairs and then completed word stems presented beneath context words in the left or right visual field. Letter-case-specific priming in stem completion was found only when the context word was the same word that had previously appeared above the primed completion word and the items were presented directly to the right cerebral hemisphere. This pattern of results was not found when participants deliberately recollected previously presented words when completing the stems. Results suggest that holistic processing, not parts-based processing as assumed in many contemporary theories of visual-form recognition, is performed in a subsystem that distinguishes specific instances in the same abstract category of form and that operates more effectively in the right hemisphere than in the left hemisphere