Exp Brain Res (1992) 89:157-171 Experimental BrainResearch 9 Springer-Verlag 1992 The effects of lighting conditions on responses of cells selective for face views in the macaque J.K. Hietanen, D.I. Perrett, M.W. Oram, P.J. Benson, and W.H. Dittrich Department of Psychology, University of St. Andrews, St. Andrews, Fife KY16 9JU, UK Received May 27, 1991 / Accepted November 30, 1991 temporal cortex Summary. Neural mechanisms underlying recognition of objects must overcome the changes in an object's ap- pearance caused by inconsistent viewing conditions, par- ticularly those that occur with changes in lighting. In humans, lesions to the posterior visual association cortex can impair the ability to recognize objects and faces across different lighting conditions. Inferotemporal lesions in monkey have been shown to produce a similar difficulty in object matching tasks. Here we report on the extent to which cell responses selective for the face and other views of the head in monkey temporal cortex tolerate changes in lighting. For each cell studied the (preferred) head view eliciting maximal response was first established under normal lighting. Cells were then tested with the preferred head view lit from different directions (i.e. front, above, below or from the side). Responses of some cells failed to show complete generalization across all lighting conditions but together as a "population" they responded equally strongly under all four lighting conditions. Further tests on sub-groups of cells revealed that stimulus selectivity was maintained despite unusual lighting. The cells discriminated between head and con- trol stimuli and between different views of the head in- dependent of the lighting direction. The results indicate that constancy of recognition across different lighting conditions is apparent in the responses of single cells in the temporal cortex. Lighting constancy appears to be established by matching the retinal image to view-specific descriptions of objects (i.e. neurons which compute ob- ject structure from a limited range of perspective views). Key words: Lighting - Face - Single cell - Temporal cortex - Macaque monkey Introduction A fundamental problem facing the visual system is the extraction of an object's form under different viewing Offprint requests to: D.I. Perrett conditions. Factors such as perspective view, object orientation, distance, movement, or lighting may produce enormous variations in the retinal image, yet the visual system is able to interpret and recognize objects correctly. One of the largest changes imposed on an object's appearance is that caused by a change in lighting which can vary in strength, direction and number of illumina- tion sources. As objects are not generally illuminated uniformly from all directions, lighting under one set of conditions can produce shading and shadows which ob- scure features visible at other times. We are unaware of the sophistication of our perceptual ability in coping with different lighting conditions because recognition is usu- ally carried out without effort. Thus in everyday life we may be aware of the end product of our recognition but we do not contemplate the effect shadows have in obscur- ing particular features visible in other circumstances. In some conditions, shadows may aid recognition rather than hinder it. Shadows can provide three-dimen- sional information about surface structure of objects and the direction of illumination. Although a few formal models for determining structure from shading have been proposed (e.g. Horn 1975; Koenderink and van Doorn 1980; Cohen and Grossberg 1984), neurophysiological and psychological evidence for the applicability of the models to human object recognition is lacking. It is a common feature of such models (e.g. Horn 1975; Pent- land 1982) that the retrieval of surface orientation and structure depends on advance knowledge of the position of the illumination source(s) and properties of the object surfaces (i.e. their reflectance etc.). Thus in general the utilization of shading information for deriving object properties would only be possible if this additional in- formation is supplied from an analysis of other aspects of the image or from memory (Ikeuchi and Horn 1981 ; Pentland 1982; Shafer 1985; Gershon et al. 1986). The effect of shadow information in provoking per- ception of three-dimensional shape is so strong that it can occur even when shadow areas have impossible (in nat- ural conditions) colours and textures, or associated movements (Cavanagh and Leclerc 1989). The only re-