Homunculus

Abstract: The prefrontal cortex (PFC) has long been thought to serve as an ‘executive’ that controls the selection of actions and cognitive functions more generally. However, the mechanistic basis of this executive function has not been clearly specified often amounting to a homunculus. This paper reviews recent attempts to deconstruct this homunculus by elucidating the precise computational and neural mechanisms underlying the executive functions of the PFC. The overall approach builds upon existing mechanistic models of the basal ganglia (BG) and frontal systems known to play a critical role in motor control and action selection, where the BG provide a ‘Go’ versus ‘NoGo’ modulation of frontal action representations. In our model, the BG modulate working memory representations in prefrontal areas to support more abstract executive functions. We have developed a computational model of this system that is capable of developing human-like performance on working memory and executive control tasks through trial-and-error learning. This learning is based on reinforcement learning mechanisms associated with the midbrain dopaminergic system and its activation via the BG and amygdala. Finally, we briefly describe various empirical tests of this framework.

Abstract: In 1937, Penfield and Boldrey published a paper of major importance.' They described their work on the effects of stimulation of the cerebral cortex in man, the procedures being carried out as exploratory manoeuvres to delineate the appropriate area for subsequent surgical intervention. They confirmed the precise topography of cortical localisation, and were able to relate stimulation of a discrete part of the brain with motor and sensory phenomena affecting a particular part of the body. Whilst these now classical studies confirmed and greatly extended what had been known from earlier observations in awake humans and from experiments in animals, the manner of presentation of their findings was remarkable. It was the first time pictorial means of illustrating cortical representation had been attempted; it was thus an entirely new concept, but it was also one which has proved a curious method of illustration and one which gives rise to a number of unforseen problems. Penfield and Boldrey set out to illustrate \"the order and comparative extent\" occupied in the sensorimotor strip.' To represent the topography of their observations, the authors departed from the rigorous textual description of the effects of stimulation of the brain and achieved an extraordinary conceptual leap: an artist, Mrs H P Cantlie, was employed to draw a sensory and motor homunculus-a term discussed below. This first homunculus to be created (fig 1) was described as giving \"a visual image of the size and the sequence of the cortical areas\". It was symmetrical in shape and illustrated both motor and sensory features together; what actually was represented, however, was ambiguous and confusing. The authors state that the size of the parts was determined \"not so much by the number of responses...but by the apparent perpendicular extent of representation of each part when these responses were multiple for the same part\". What is meant by \"perpendicular\" is not stated, and it becomes even less clear from the subsequent comment: \"that the large size of the thumb and the lips indicates the vertical extent of Rolandic cortex devoted to those parts in individual cases is very large\". It is unclear whether \"vertical\" is the same as \"perpendicular\", and whether these terms mean longitudinal over the cortical surface, or deep-since the authors also stimulated deeper layers of the brain after having resected areas for extirpation of abnormal brain. The referral to responses which were multiple for the same part is also difficult to interpret, since it could imply multiple sites were effective in eliciting responses, or that only in certain areas were multiple sites actually stimulated. Whilst the homunculus appears to relate to length rather than width, the parts illustrated are in fact enlarged in both dimensions. Also noteworthy is that responses have been transposed to the same side of the brain for the purposes of representation, and it is not possible from the illustration to distinguish unilateral from bilateral effects of cortical stimulation. Thirteen years later, in their monograph entitled The cerebral cortex of man, Penfield and Rasmussen made another attempt to illustrate cortical representation.2 This was preceded by a diagram of a cross-section of the cere-

Abstract: Fine-scale somatotopic encoding in brain areas devoted to sensorimotor processing has recently been questioned by functional neuroimaging studies which suggested its absence within the hand area of the human primary motor cortex We re-examined this issue by addressing somatotopy both in terms of functional segregation and of cortical response preference using oxygenation-sensitive magnetic resonance imaging at high spatial resolution In a first step, spatial representations of self-paced isolated finger movements were mapped by using motor rest as a control state A subsequent experimental design studied the predominance of individual finger movements by using contrasting finger movements as the control task While the first approach confirmed previous reports of extensive overlap in spatial representations, the second approach revealed foci of differential activation which displayed an orderly mediolateral progression in accordance with the classical cortical motor homunculus We conclude that somatotopy within the hand area of the primary motor cortex does not present as qualitative functional segregation but as quantitative predominance of certain movement or digit representation embedded in an overall joint hand area