Chromatic processing in Hemianopia

When observers have to respond to two simultaneously presented targets, their reactions times are significantly faster than responses to single targets. This is termed the Redundant Target Effect (RTE). In experiment one, we demonstrated the presence of RTE in ten normal observers using, luminance-defined, colour-defined and combined luminance-colour defined stimuli, even when targets were presented in different hemifields. For colour-defined targets, Random Luminance Modulation (RLM) technique was used to mask any luminance cues. In experiment two, we applied the same technique to investigate the extent of chromatic processing within the field defect of a group of cortically blind patients. Previously, RTE had only been consistently reported in 1 out of 20 cortically blind patients tested, using combined luminance-colour targets (Marzi, Tassinari, Aglioti & Lutzemberger, 1986). Here we show evidence for chromatic processing within the field defect in all 5 cortically blind patients tested, by showing that the reaction times to sighted field presentations are affected if a similar target is presented within the field defect. More specifically, depending on the stimulus conditions all five patients illustrated either facilitation (RTE, i.e. shorter reaction times) or inhibition (i.e. slower reaction times) of chromatic processing. Overall the pattern of facilitation and inhibition for luminance and colour defined targets are complex. However, s-cone sensitive stimuli consistently led to an inhibitory effect in all five patients. This finding is significant as s-cone afferent signals reportedly, do not project to midbrain structures such as superior colliculus, often implicated in blindsight.