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Independent Effects of Colour on Object Identification and MemoryToby J. Lloyd-Jones1 and Kazuyo Nakabayashi2

1Swansea University2University of Kent

Correspondence to: Toby J. Lloyd-Jones, Department of Psychology, Swansea University, Singleton Park, Swansea, SA2 8PP, Wales, UK. Phone: +44 (0)1792 602420. Email: T.J.Lloyd-Jones@swansea.ac.uk.

Short title: Colour and object processingAbstract

We examined the effects of colour on object identification and memory using a study-test priming procedure with a coloured-object decision task at test (i.e., deciding whether an object is correctly coloured). Objects were selected to have a single associated colour and were either correctly or incorrectly coloured. In addition, object shape and colour was either spatially integrated (i.e., colour fell on the object surface) or spatially separated (i.e., colour formed the background to the object). Transforming the colour of an object from study to test (e.g., from a yellow banana to a purple banana) reduced priming of response times, as compared to when the object was untransformed. This utilisation of colour information in object memory was not contingent upon colour falling on the object surface or whether the resulting configuration was of a correctly or incorrectly coloured object. In addition, we observed independent effects of colour on response times, whereby coloured-object decisions were more efficient for correctly as compared with incorrectly coloured objects but only when colour fell on the object surface. These findings provide evidence for two distinct mechanisms of shape-colour binding in object processing. Keywords: Object, Colour, Identification, Priming, New-association Priming, Memory

Acknowledgements

This work was supported by Leverhulme Trust award F/00236/H to Toby Lloyd-Jones.

Colour often plays an important role in our everyday interactions with objects, for instance in quickly and efficiently identifying particular fruit and vegetables at the supermarket, deciding whether a fruit is ripe or stopping at red traffic lights. These events require the perception and processing of colour and they also require combining colour with other forms of information associated with a particular object or class of objects, primarily information about object shape. The focus of the present article is on how information about object colour and shape is combined in object identification and object memory. In particular, we propose that different binding mechanisms mediate the influence of colour on these two abilities.

Concerning object identification, there is general agreement that colour provides a perceptual input to early stages of visual processing (e.g., Davidoff, 1997; Wurm, Legge, Isenberg & Luebker, 1993). In addition, colour may be represented at a perceptual level in a structural representation system (Price & Humphreys, 1989; Tanaka, Weiskopf & Williams, 2001) and/or at a semantic level where stored conceptual knowledge of prototypical object colour provides an associative link between a representation of object shape and the object name (Davidoff, 1991, 1997; Tanaka et al., 2001). Stored knowledge of object colour likely plays a greater role in object identification than presence of colour in the display (Joseph & Proffitt, 1996; Mapelli & Behrman, 1997).

Consistent with these proposals, a number of studies have shown that colour improves identification when it is appropriate for the object (e.g., Humphrey, Goodale, Jakobson & Servos, 1994; Joseph & Proffitt, 1996; Price & Humphreys, 1989; Tanaka & Presnell, 1999; Vernon & Lloyd-Jones, 2003; although see Biederman & Ju, 1988). Of most relevance here, Price and Humphreys (1989) have also demonstrated that the influence of colour on object naming is only apparent when colour occupies the internal surfaces of objects: it does not benefit performance when presented as the background to the object. They conclude that colour effects on object identification are contributed by a representation in which colour and shape are in some way conjoined (p.819). They suggest that there are independent but richly interconnected representations of object colour and shape, whereby activation of a strongly associated colour representation (e.g., yellow) in addition to activation of the shape representation (e.g., banana) leads to more efficient retrieval. In contrast, the role of colour in object memory has been little studied (e.g., Cave, Bost & Cobb, 1996; Lloyd-Jones, 2005; Mecklenbrauker, Hupbach & Wippich, 2001, Vernon & Lloyd-Jones, 2003; Zimmer & Steiner, 2003). Nevertheless, recent studies suggest that when colour is made relevant to task performance it can contribute to object memory (Lloyd-Jones, 2005; Vernon & Lloyd-Jones, 2003). In particular, Vernon and Lloyd-Jones (2003) assessed effects of colour transformation between study and test, from a correctly to incorrectly coloured object (e.g., from a yellow banana to a purple banana) and vice versa, on naming and coloured-object decision (i.e., deciding whether an object is correctly coloured). Colour transformation substantially reduced priming for both correctly and incorrectly coloured objects in coloured-object decision but not naming. They propose that stored colour information is retrieved to aid memorial performance when colour information is required by the task (i.e., in coloured-object decision; for a similar argument, see Holbrook, Bost & Cave, 2003).

Vernon and Lloyd-Jones (2003; Lloyd-Jones, 2005) also argue that the priming effects they observed likely reflect some degree of new-association priming: memorial systems are able to bind together arbitrarily different stimuli into a representation that benefits performance (e.g., Goshen-Gottstein & Moscovitch, 1995a, 1995b; Micco & Masson, 1991; Musen & ONeill, 1997). Musen, Szerlip and Szerlip (1999) have examined some of the parameters of this phenomenon. In particular, whereas previous studies had only used stimuli in which the two elements to be associated were physically integrated, Musen et al., found new-association priming for spatially separated words and colours but not for spatially separated abstract shapes and colours or nonwords and colours. They concluded that when stimuli are either familiar (e.g., words rather than nonwords) or perceptually integrated new-association priming may be observed. Thus, under certain circumstances pre-existing memorial representations may support new-association priming between spatially separated stimuli.

In summary, it appears that two distinct mechanisms may mediate shape-colour binding in on-line object identification and object memory, respectively: (a) effects of colour on object identification are mediated by an input representation which combines object shape and associated colour; and (b) effects of colour on object memory are mediated by a form of new-association priming. To test this proposal, we examined the effects of colour on coloured-object decision using a basic study-test priming procedure. We manipulated both stimulus colour and spatial integration of object shape and colour. Objects were selected to have a single colour which was associated with object identity (e.g., yellow banana or red fire engine) and which we expected would maximise effects of colour, if present (e.g., Tanaka & Presnell, 1999). The stimuli were either correctly or incorrectly coloured. Furthermore, object shape and colour could be either spatially integrated (i.e., colour fell on the surface of the object) or spatially separated (i.e., colour formed the background to the object). Manipulations of object colour were carried out across study-test phases so that at test participants were presented with a stimulus that remained the same as at study, a stimulus that changed its perceptual appearance from study to test, or a new (not previously encountered) stimulus which served as a baseline against which to measure priming.

We predicted that: (a) there would be reduced priming for transformed stimuli, demonstrating that colour is normally utilised by the system mediating performance; (b) if new-association priming benefits memorial performance then priming of equivalent strength should be observed for correctly and incorrectly coloured objects and across spatially integrated and spatially separated conditions; (c) if a different form of colour-shape binding benefits object identification, then we should observe more efficient processing of correctly coloured as compared with incorrectly coloured objects in coloured-object decision but only under spatially integrated conditions: these effects should be independent of any effects of colour on priming; and finally (d) we note that it is possible that the same mechanism mediates effects of colour in both on-line identification and priming. In this case, we would expect similar effects of colour on object identification and priming to be observed, but only under spatially integrated conditions.METHODParticipants. Two hundred and thirteen undergraduate students at the University of Kent participated in this study for course credit. All had normal or corrected-to-normal visual acuity and none were colour-blind.Materials / Apparatus. The initial pool of stimuli were colour photographs of 75 objects from a number of different categories. Most pictures were taken from a photographic website (www.PhotoObjects.net) and a subset was selected via an internet image search using the Google search engine. Forty-three of the objects were in the list of object drawings used in Snodgrass & Vanderwart (1980). The objects were selected on the basis that each object had a single associated colour, where the surface colour of each object was based on colour agreement scores obtained by Joseph (1997) and Vernon & Lloyd-Jones (2003).

We used the imaging software Adobe Photoshop CS2 to create 3 versions of

each object; a correctly coloured object, a greyscale object, and an incorrectly

coloured object. Correctly coloured objects were converted to greyscale using greyscale mode which preserves luminance (i.e., L*, the lightness component1) whilst discarding colour. In line with Vernon & Lloyd-Jones (2003) and Lloyd-Jones (2005), to convert correctly coloured objects to incorrectly coloured objects we rotated the correct colours across objects whilst ensuring that correctly and incorrectly coloured objects were matched for colour frequency and luminance. Thus, an incorrectly coloured object was created by selecting the surface colour of a correctly coloured object and pasting the colour onto a different object using the colour replacement tool. The luminance of the colour-replaced object was adjusted using the brightness contrast tool. Mean values (and standard deviations) of stimulus luminance were: correctly coloured=163 (9.7), incorrectly coloured=160 (7.5), greyscale=146 (9.9). We then adopted two procedures to select the most appropriate stimuli for the experiment. First, we established that there were clear benefits of colour on object naming. Second, we examined colour agreement between the surface colour of each object (i.e., the colour that was assigned by the experimenters) and

participants knowledge of the prototypical colour of each object: we selected those objects with the highest agreement. The procedures are described below.1. Object naming task. There were 21 participants. A within-participants design was used with surface colour (correct/greyscale/incorrect) as the variable. The task was to name each object aloud as quickly as possible. Each trial began with a fixation cross for 250ms followed by a 500ms blank screen and then by the object. The object remained on the screen until a response had been made. There were 25 trials per condition per participant, with no object encountered more than once, but by rotation all 75 stimuli were encountered in each condition across all participants. The order of the trials was randomised. The dependent measures were response time (RT) and accuracy. Results were analysed by-participant and by-item (subscripts 1 and 2 refer to by-participant and by-item analyses, respectively). The within-participants ANOVA showed a main effect of colour, F1(2,40)=7.22, p