Anatomical organisation of the primate postcentral gyrus has been described in terms of several different cytoarchitectures.1 2 Powell and Mountcastle stated that the area 3 was a typical koniocortex with granular cells, whereas in areas 1 and 2 the morphological characteristics changed gradually to the homotypical parietal association cortex in the monkeyMacaca mulatta.1 Iwamuraet al reported the physiological correlates on the anatomical rostrocaudal axis in monkeys.2 The ratio of skin neurons to total neurons was the largest in area 3b and decreased gradually toward the caudal part of the postcentral gyrus.2 Specific types of stimulation such as rubbing of the skin in certain directions were effective in activating some of the caudal part of the postcentral gyrus. The anatomical and physiological data in the primate lead to the reasonabe hypothesis that there is a rostrocaudal functional gradient within the postcentral gyrus. This notion may explain why a lesion in the postcentral gyrus causes varied sensory disturbance in various people.

A 49 year old right handed man suddenly developed dysaesthesia in the right hand. This recovered gradually, but 1 month later he still had an impaired tactile recognition for objects. His voluntary movements were skillful. Deep tendon reflex was slightly exaggerated in his right arm. Babinski's sign was absent. His language was normal. Brain MRI on the 35th day after the onset showed a laminar necrosis on the caudal edge of the lateral portion of the left postcentral gyrus (figure).

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Brain MRI of the patient. Transaxial T1 weighted (above) and T2 weighted images (below) are shown. The T1 weighted image disclosed a high intensity lesion distributed laminarly in the caudal edge of the left postcentral gyrus (Brodmann 1–2). Arrows indicate the left central sulcus.

Somaesthetic assessment was done during the 21–28th days of the illness.

Elementary somatosensory functions were assessed, including light touch (long fibre cotton), pain (pinprick), thermal sensation (cold and hot water), joint position sense (tested by the ability of the patient to identify flexion or extension of fingers with closed eyes), and vibration sense (128-Hz tuning fork).

Intermediate somatosensory tasks were carried out. For two point discrimination, the examiner placed a pair of plastic needles of a slide caliper on the index finger pad of the patient, who had his eyes closed, and asked him to answer the number of touched needles, “one” or “two”. For tactile localisation the examiner touched a point on the right or left hand of the patient, who had his eyes closed, with a brush, then asked him to indicate the point by touching the place with the first finger of the counter hand. For weight perception, the patients were asked to arrange the stimuli in a correct order of the weight with either the left or right hand. The stimuli were six metal plates of equal size, shape, and texture weighing 50, 60, 70, 80, 90, and 100 g. For texture perception, we prepared six wooden plates of an identical size and shape, on which one of six different textures (sandpaper, felt, wood, wool, fine grain, synthetic rubber) were mounted. The patient palpated one texture by either hand with his eyes closed. Then he was asked to select tactually a correct one among the six textures. For shape perception (three dimensional figures) the patient palpated one of the five wooden objects (cylinder, cube, sphere, prism, and cone) with his eyes closed. Then he was asked to explain the shape verbally. For extinction, the examiner delivered light and brief tactile stimuli, using the tips of the index fingers, to the dorsal surface of left, right, or both hands of the patient.

For tactile object recognition, the 15 objects that are used in the naming list of the Western aphasia battery test were presented to either hand. For naming of objects, the patient was asked to name a single manipulated object. In matching of objects, the patient first grasps a single object among a selection of five objects, and then he was asked to select the correct object among the five.

In elementary sensory function, the test for light touch, pain, thermal sensation, joint position sense, and vibration sense demonstrated no abnormalities for both hands. The results of intermediate sensory tests showed that in all tests, except for shape perception, we could detect no disturbance in both hands. He could not discriminate the shape with his right hand. The correct responses were 5/5 with the left hand and 0/5 with his right hand. The correct responses in the tactile naming test were 2/15 for the right hand and 15/15 for the left hand. The correct responses of the tactile-tactile matching test was 4/15 with his right hand and 15/15 with his left hand. So the abilities of tactile recognition and tactile-tactile matching were disturbed with the right hand.

According to Delay, disturbances of the tactile process in the cortex are classified into at least three types.3 Ahylognosia is a distubance in the ability to discriminate materials. Amorphognosia is a disturbance in the differentiation of forms. Tactile agnosia is the inability to recognise the identity of objects in the absence of ahylognosia and amorphognosia. In Delay's terms, our patient showed amorphognosia but not tactile agnosia. Iwamura and Tanaka suggested that the hand region of area 2 in the rhesus monkey is concerned with the tactile perception of the discrimination of certain object forms.4 The lesion localised at the equivalent cortical region. This region thus may be critical for the tactile discrimination for shape.

Rich intrinsic corticocortical connections are demonstrated within the rhesus monkey's postcentral gyrus, starting from Brodmann area 3b and projecting to areas 1 and 2.5 This corticocortical connection may be a main route of inputs to area 2. This suggests that within the postcentral gyrus somatosensory information is processed from primary sense reception to integrating and more associating stages. The results from our patient are compatible with the notion that in the caudal portion of the human postcentral gyrus the more complex process such as shape perception is processed.