Introns

Illusions as neuro-signsNicholas Wade

One of the reasons visual scientistsfind illusions fascinating is that theirpictorial simplicity belies thecomplexity of visual processing.Illusions can direct us towardsneurophysiological interpretations atmany levels, which I have called‘neuro-signs’. The most useful visualillusions are those that point to thesimplest neural level possible, that is,those that provide signs of neuralprocessing at early stages of the visualsystem, such as in the retina itself.

The region of the retina that canchange the activity of nerve cellsthroughout the visual system whenlight falls on it is called the ‘receptivefield’ of those nerve cells. Primateretinal ganglion cells have receptivefields that are concentric andantagonistic — if the centre of a

receptive field is excited by light thesurround is inhibited (called‘surround inhibition’), or vice versa.

Are there any visual signs of suchneural antagonism? There could be.In the black and white grid patternshown below, a variety of illusorydots can be seen at the intersectionsof the black and white grid. In theupper half, dark grey dots appear atthe white intersections between theblack rectangles, although a lightmeter would not register sucheffects. This is called a Hermanngrid; its reverse in the lower half,where illusory light grey dots areseen in the black intersections, iscalled a Hering grid.

In both cases, the dots at anintersection disappear as soon as it isfixated, but they remain at thosenearby. The dots are more difficult tosee at the smallest intersections inboth grids. Because of this effect, it isthought the illusory dots might berelated to the distance from thecentre of the retina (or ‘retinaleccentricity’); receptive fields havebeen found to be much smalleraround the centre of the retina thanmore peripherally. Indeed, theoptimal separation for producing theillusory dots increases with retinaleccentricity in a similar manner tothe increase in receptive field size.

But why do the illusory dotsappear at all? One suggestion is thatconcentric receptive fields of thesame dimensions will be stimulateddifferently when they fall at anintersection between four rectanglesthan when flanked by two rectangles— there would be more surroundinhibition at an intersection thanelsewhere. This would make a lightintersection darker than itsneighbours and a dark intersectionlighter. Such an interpretation doesnot account for the dots that arevisible between rectangles that aresimply outlines, although these dotsare not as compelling.

When coloured rectangles areplaced on a grey background (seeabove), the illusory dots at theintersections take on the colour of the

rectangles: they appear to be lightgreen in the grey intersections of theupper half, and reddish in the greyintersections of the lower half. This isbecause some receptive fields at theretinal ganglion cell level displaycolour opponency — the centre canbe excited by, say, red and thesurround inhibited by green; othersoperate on blue–yellow opponency.

Hermann–Hering grids providesigns of a link between visualperception and neurophysiology.The link is made at an early stage ofvisual processing — the differencesin the activity of the retinal ganglioncells — with the assumption that theneural signals for these differencesare preserved in their transmission tothe visual cortex of the brain.

Address: Department of Psychology,University of Dundee, Dundee DD1 4HN, UK.

Magazine R439

This is the second in a short series ofarticles by Nicholas Wade, describingsome common visual illusions.