The parietal cortices participate in encoding, short-term memory, and decision-making related to tactile shape.

We routinely identify objects with our hands, and the physical attributes of touched objects are often held in short-term memory to aid future decisions. However, the brain structures that selectively process tactile information to encode object shape are not fully identified. In this article we describe the areas within the human cerebral cortex that specialize in encoding, short-term memory, and decision-making related to the shape of objects explored with the hand. We performed event-related functional magnetic resonance imaging in subjects performing a shape discrimination task in which two sequentially presented objects had to be explored to determine whether they had the same shape or not. To control for low-level and nonspecific brain activations, subjects performed a temperature discrimination task in which they compared the temperature of two spheres. Our results show that although a large network of brain structures is engaged in somatosensory processing, it is the areas lining the intraparietal sulcus that selectively participate in encoding, maintaining, and deciding on tactile information related to the shape of objects.

Integrating Somatosensory Information Over Time.

Our choices are often informed by temporally integrating streams of sensory information. This has been well demonstrated in the visual and auditory domains, but the integration of tactile information over time has been less studied. We designed an active touch task in which participants explored a spheroid-shaped object to determine its inclination with respect to the horizontal plane (inclined to the left or the right). In agreement with previous findings, our results show that more errors, and longer decision times, accompany difficult decisions (small inclination angles). To gain insight into the decision-making process, we used a time-controlled task in which the experimenter manipulated the time available for tactile exploration on a trial-by-trial basis. The behavioral results were fit with a bounded accumulation model and an independent sampling model that assumes no sensory accumulation. The results of model fits favor an accumulation-to-bound mechanism and suggest that participants integrate the first 600 ms of 1800 ms-long stimuli. This means that the somatosensory system benefits from longer streams of information, although it does not make use of all available evidence.

Tactile object categories can be decoded from the parietal and lateral-occipital cortices.

The visual system classifies objects into categories, and distinct populations of neurons within the temporal lobe respond preferentially to objects of a given perceptual category. We can also classify the objects we recognize with the sense of touch, but less is known about the neuronal correlates underlying this cognitive function. To address this question, we performed a multivariate pattern analysis (MVPA) of functional magnetic resonance imagining (fMRI) activity to identify the cortical areas that can be used to decode the category of objects explored with the hand. We observed that tactile object category can be decoded from the activity patterns of somatosensory and parietal areas. Importantly, we found that categories can also be decoded from the lateral occipital complex (LOC), which is a multimodal region known to be related to the representation of object shape. Furthermore, a hyperalignment analysis showed that activity patterns are similar across subjects. Our results thus indicate that tactile object recognition generates category-specific patterns of activity in a multisensory area known to encode objects, and that these patterns have a similar functional organization across individuals.