Brain Research, 342(1985)391-395 391 Elsevier BRE 21022 Somatosensory cortex in macaque monkeys: laminar differences in receptive field size in areas 3b and I MRIGANKA SUR, PRESTON E. GARRAGHTY and CHARLES J. BRUCE Section of Neuroanatomy, Yale University School of Medicine, New Haven, CT06510 (U.S.A.) (Accepted April 16th, 1985) Key words: somatosensory cortex - - cortical layer - - receptive field size We have examined receptive field sizes of neurons in granular, supragranular and infragranular layers within somatosensory corti- cal areas 3b and 1 in macaque monkeys. Receptive fields of neurons in layer 4 are smaller than receptive fields of neurons above or be- low layer 4. In addition, neurons in area 1 have larger receptive fields than neurons in corresponding layers of area 3b. Few studies have examined physiological prop- erties of neurons associated with interlaminar pro- cessing of information in the somatosensory cortex of cats 3-14 or monkeys 26. In the present study, we chose to examine a relatively simple feature of neuronal re- ceptive fields, receptive field size, as a function of cortical layer in areas 3b and 1 of postcentral somato- sensory cortex in macaque monkeys. There were at least two reasons to undertake this study. First, com- pared to the elaboration of receptive field properties as a function of cortical laminae in primary visual cor- tex or area 17 (ref. 8,9), receptive field properties re- main relatively uncomplicated in somatosensory cor- tical area 3b. Thus, most neurons in area 3b have ho- mogeneous receptive fields without separate on and off zones 2-~. Even in area 1, only a few cells have sep- arate on and off zones or show other properties such as orientation selectivity and directionality2.7,10,1x,13. ~8,22. We reasoned that one feature of receptive fields that could be expected to show some change with cor- tical laminae in areas 3b and 1 would be receptive field size. A second reason, related to the first, arose from a recent suggestion by Mountcastle 17 in attempting to explain differences in conclusion between research groups regarding body representations within areas 3b and 1. Mapping experiments by Merzenich et al. 16 in owl monkeys, suggested that each of areas 3b and 1 contains a complete and separate representation of the body surface. A key observation leading to this conclusion was that receptive fields on specific parts of the skin surface (the hand, for example) reversed in sequence at the border between areas 3b and 1. Similar observations were later made in cynomolgus monkeys 19, squirrel monkeys 24 and cebus mon- keys 6. In contrast, McKenna et al. 15 reiterated conclusions reached in earlier studies 4.25 that there was only a single representation of the skin surface overlying areas 3b, 1 and portions of area 2. Impor- tantly, McKenna et al. t5 failed to consistently find re- versals in receptive field sequences on the hand as they traversed the border between areas 3b and 1. Mountcastle, in a comprehensive review 17, suggested that one reason for these differences in observations and conclusions could be that while Merzenich et al. 16 sampled activity in or in the vicinity of the mid- dle, thalamic recipient layers of cortex, McKenna et al.15 sampled all layers of cortex and treated recep- tive fields from all laminae 'as equivalent for the pur- poses of map construction'. Clearly, if receptive fields changed in size as a function of cortical layer, pooling data collected from different layers could mask receptive field reversals at borders of cortical areas and even produce inconsistencies in receptive field sequences. While Mountcastle concluded that Correspondence." M. Sur, Section of Neuroanatomy, Yale University School of Medicine, 333 Cedar Street, New Haven, CT 06510, U.S.A. 0006-8993/85/$03.30 © 1985 Elsevier Science Publishers B.V. (Biomedical Division)