Information available for the STS to determine the expectedness of a stimulus

In the previous chapter (3) the cellular properties and functions of areas involved in somatosensory processing were reviewed in conjunction with the anatomical connections between these areas. They were discussed in relation to two parallel tactile processing pathways involved with the tactile discrimination and recogniton of objects and the spatial analysis of objects through the somatosensory and visual modalities. No position in these pathways was considered for the STS mainly because of the scant nature concerning the dimensions of somatosensory processing within this area - and the largely speculative account of its overall functions.

The present study has described cells in the STS whose activity can best be interpreted as discriminating between expected and unexpected tactile stimuli. To understand the response selectivity of these cells it is essential first to consider the information that would be needed to establish such selectivity. It is then helpful to speculate on the potential sources of this information from the functional properties of areas of the somatosensory and association cortex and to assess the plausibility of such areas as routes via which the appropriate information could be made available to the STS. Placing the STS in one or both of the proposed tactile pathways at a level of processing appropriate to the properties of the cells and the function of the STS as a whole should then be possible.

STS for deriving the expectedness of a tactile stimulus, it will be useful to recap on the features necessary for this. It is essential to have: (1) sensory information about the nature of the stimulus, i.e. its shape, size and texture; (2) sensory motor feedback to indicate the extent of movements executed and hence the likelihood of encountering objects of remembered spatial positions; (3) associative memories based on sensory experiences from which predictions are set up which may or may not be matched; (4) visual feedback in passive tactile stimulation indicating the proximity of a stimulus to the tactile receptive field; and (5) a spatial representation of the monkey's immediate environment, i.e. the location in space of all objects in the monkey's extrapersonal space (at one moment in time) relative to the monkey's body axis.

These requirements will be considered point by point:-

(1) The Sl-SII-insular cortex tactile processing pathway (Mishkin, 1979; Murray and Mishkin, 1984; Murray et al, 1980) attributed with a functional role in tactile discrimination and recognition of objects would seem to be the best route by which neurones in the STS could be supplied with the appropriate information about the sensory nature of the stimulus, e.g. its size, shape, roughness, hardness, etc. There is now anatomical evidence to support this claim, as Friedman et al (1986) have demonstrated a reciprocal connection between the dysgranular fields of the insular cortex and both the upper and lower banks of the STS cortex. Another sensory route for this information may be through area 7 of the posterior parietal association cortex which itself receives projections from both SII and the insular cortex

(2) The sensory motor feedback necessary for STS cells to determine what motor movements would result in tactile collision with a 'known' object in space could arise from the following systems. Numerous studies of neurones in the posterior parietal cortex report high involvement of cells in the active movements of the monkey (see Chapter 3), in particular of visually guided (area 7) and somaesthetically guided (area 5) movements to objects in the monkey's extrapersonal space. Such information from the Sl-area 5-area 7 pathway could adequately provide the STS neurones with the appropriate sensory motor feedback, and projections from area 7 (both regions 7a and 7b) do project to the upper bank of the STS (Seltzer and Pandya, 1978, 1984).

(3) Mishkin (1982) postulates that coded representations of objects are stored in the association areas of the cortex whenever stimulus

activation of these areas also triggers a

cortico-limbo-thalamo-cortical circuit. Once established, he suggests that the stored central representation can enter into association with a variety of other stored representations (sensory, affective, spatial, motor) via reciprocal connections with the relevant structures. Hence the associative memories necessary for defining the expectedness of a stimulus could be established within the STS association cortex by virtue of its reciprocal connections - sensory associations via the insular (tactile) and inferotemporal (visual) cortices, spatial and motor through the posterior parietal cortex, and affective (and crossmodal) associations via the amygdala (see Chapter 3).

(4) Integration between visual and tactile stimulation such that would indicate the proximity of an object (whether stationary or moving) to the tactile receptive field has been reported in the posterior parietal association cortex, predominantly in area 7 neurones. The visual information requisite for the STS cells to derive the expectedness of a tactile stimulus in passive stimulation could therefore emanate once more from the SI~area5-area 7 tactile-visual pathway. As previously described, efferent projections from area 7 to the STS have been reported (e.g. Seltzer and Pandya, 1978, 1984) but whether these projections are for visual, somatosensory or integrated visual-somatosensory information is still unknown.

(5) Finally, the STS cells require information relating to the spatial location of objects in the monkey's immediate environment with reference to the monkey's body axis. Although functions attributed to the posterior parietal cortex as a whole include the formation of a spatial representation of the monkey's body and extrapersonal space, this is primarily for guiding motor acts to targets of interest (e.g. Mountcastle et al, 1975). The physical location of objects in space is more a memory function, perhaps attributable to the amygdala/hippocampal complex with which the STS has direct and indirect connections (e.g. Aggleton et al, 1980).

4.4.5 A place for the STS in tactile processing