Population coding of affect across stimuli, modalities and individuals
Junichi Chikazoe, Daniel Lee, Nikolaus Kreigeskorte & Adam Anderson, Universities of Cornell, Toronto and Cambridge. Nature Neuroscience, vol. 17, No. 8, August 2014 (http://www.nature.com/natureneuroscience)
Summary and review of the above paper
INTRODUCTION: Studies of neural activity in response to both complex visual scenes and also tastes revealed the neural code for a continuous axis of pleasant to unpleasant emotional values (valence). The medial and lateral orbitofrontal cortices (OFC) supports a valence code that is independent of the sensory origin of signals. This allows the attractive or aversive qualities of signals to be quantified regardless of the nature or modality of the stimuli.
Function by brain region
Low-level features can be decoded in the early visual cortex (EVC). Higher-level activities such as object categories have been detected in the ventral temporal cortex (VTC). However, lesion and neuroimaging studies suggest a central role in emotional or affective processing of visual, taste or olfactory signals for the lateral and medial orbitofrontal. Increased neural activity in the medial orbitofrontal and the adjacent ventromedial prefrontal is seen to be related to the value of goals, reward expectancy, outcome value and positive emotional outcomes.
However, there is found to be considerable regional overlap between positive and negative emotional outcomes. Neurons related to positive outcomes, negative outcomes and both positive and negative outcomes can be interspersed in the same area. The majority of the emotionally sensitive regions in the medial and lateral orbitofrontal are responsive to both positive and negative emotion.
In observations of the early visual cortex (EVC), ventral temporal cortex (VTC) and the orbitofrontal, it was found that activity in the EVC correlated more to visual features, activity in the VTC correlated more to categories such as degree of life-like animation or particular object categories, and that activity in the orbitofrontal correlated more to emotional response (valence). The neurons in this area were organised in a continuous range of positive and negative response, rather than there being specialised positive and negative regions. The orbitofrontal and ventromedial regions responded equally to positive and negative valence. Complex visual scenes are transformed in the orbitofrontal and related areas into neuron populations coding emotional response.
Independence of modality
It was noted that visual scenes that differed in both physical features and animacy, could nevertheless give rise to similar subjective emotional responses in the orbitofrontal. This suggests that emotional coding can be distinct from the object properties of sensory cortices, such as those of the ventral temporal cortex. Other studies showed a similar posterior to anterior brain axis. Visual features were mainly represented in the EVC, animacy in the VTC and emotional affect in the ventromedial prefrontal (vmPFC) including the medial and lateral orbitofrontal.
One study tested for commonality of emotional responses, independent of which of the sensory modalities it might come from; in this case testing was between visual and taste senses. The response of the orbitofrontal was seen to be independent of such modality origins. So called supramodal responses, that is emotional responses to both taste and visual signals, were apparent in the medial and lateral orbitofrontal, the ventromedial prefrontal and the midcingulate cortex. The study also compared the similarity of neural activity in different subjects’ brains when viewing the same signals. For the ventral temporal cortex the similarity of activity was around 80%, but in the orbitofrontal it was only a bit over 50% indicating greater emotional variation between subjects.
Emotion in the sensory cortex
However, emotional response/affect is also moderately represented in the ventral and anterior temporal cortex. Representations of emotion are thus seen to be neuron population codes in particular regions. The temporal and insula cortices responded to particular sensory modalities, whereas the lateral and medial orbitofrontal had responses that spanned different modalities. While the orbitofrontal and related areas appear to be the main centre of emotional activity, the study did suggest some emotional response with the ventral temporal cortex. But low-level visual properties and emotional response did not arise from the same areas of activity but from distinct regions of activity in the ventral temporal cortex. Moreover, where there was emotion/affect in the ventral temporal cortex, it was concentrated in the temporal anterior ventral and polar areas, which have dense connections to the orbitofrontal.
The common neural currency
It has been suggested that a common scale is needed for organisms to assess the relative value of different reward/punishers signals such as availability of water, availability of food or presence of predators. The orbitofrontal has been shown to be vital for generating this common scale or common neural currency. Even when signals in the orbitofrontal were from one modality, they produced the same type of response as signals spanning different modalities. The lateral and medial OFC and the ventromedial all provided this type of response. The medial OFC is seen as more excitatory and the lateral as more inhibitory. Thus same valence information may be used to excite or inhibit.
A question unanswered
As usual in neuroscience, we have travel the final mile by ourselves. No answer is suggested to how the orbitofrontal and related areas create a common currency from anything as disparate as a visual scene and the taste of food or wine.