PMC

Examples of colouring-in done by synaesthete AD when asked to demonstrate where her synaesthetic colours appeared. The outlined black areas were drawn by the experimenter and presented to AD and she was then asked to place the colours exactly as she saw them.

Features such as colour and shape (letters in this figure) can recombine to form an illusory conjunction in perception. Illusory conjunctions can be corrected for guessing by using intrusion errors (for example, the report of another letter or colour in the stimulus set). Intrusion errors seldom occur, for either normal perceivers or patients with spatial deficits. More elaborate corrections for guessing have been proposed (REF. ), but true illusory conjunctions, like the example shown, have still been supported.

Detection of a red X among green distractors is easy when colour or shape alone can be used to discriminate the target from distractors, whereas detecting the red X among green Xs and red Os is more difficult. For the feature search display feature maps that detect colour and those that detect shape need not interact to perform the task, while in conjunction search they must. The time taken to detect a feature is typically independent of the number of distractors (as the target ‘pops out’) whereas the time taken to detect a conjunction increases linearly as the number of distractors increases. According to feature integration theory conjunction search is more difficult because spatial attention is needed to co-localize the two features.

First, the colours that were induced by the numbers 2 and 7, for each of the two synaesthetes, were established by having them adjust colours on a computer screen to match their synaesthetic colour. In the study, two achromatic digits were displayed. Two-hundred milliseconds later, four ‘target’ dots were presented that were either the colour induced by the 2s or the colour induced by the 7s and were different for each synaesthete. Dot colour was either consistent or inconsistent with the induced colour of the digit. The digits were irrelevant for the task and were always located 8° to the left and right of fixation. Only the target dots varied in location. For both synaesthetes, naming the colour of the dots took longer when they were inconsistent with the induced synaesthetic colour than when they were consistent, but more importantly this difference was greater when attention was distributed widely over the visual field (165 ms on average) than when it was focused narrowly (52 ms on average). The digits were more likely to induce synaesthesia when they were located within, rather than outside, the spatial area being monitored.