Academic journal article Canadian Journal of Experimental Psychology

Stimulus-Response Compatibilities during Top-Bottom Discriminations

Academic journal article Canadian Journal of Experimental Psychology

Stimulus-Response Compatibilities during Top-Bottom Discriminations

Article excerpt

Participants indicated whether a small dot was located near the top or bottom pole of a rotated object. Response times increased as a function of object orientation more for top trials than for bottom trials. The interaction between orientation and response was shown to be due to a relationship between response times and the dot's height on the screen. The orientation effect was influenced, positively and negatively, by a vertical arrangement of the response keys depending on whether the upper or lower key was used for the top response. Horizontal key placement produced an intermediate orientation effect, with asymmetries of about 180° depending on which hand was used for top responses. This task appears to reflect spatial stimulus-response compatibilities more than object processing.

Keywords: orientation, object identification, top-bottom discriminations, mental rotation, stimulus response compatibility

When participants are required to indicate whether a small dot is located near the top or bottom pole of a rotated object, their response time increases as a linear function of the orientation away from the upright orientation of the depicted object. This description of the response time function tends to suggest that the increase in response times is somehow related to the processing of the object stimulus and its orientation, such as a delay in assigning the top to the stimulus (Rock, 1973). Alternatively, it has been suggested that mental rotation may be required to determine the orientation of a stimulus (de Caro, 1998), which would then suggest that the response time pattern shown during the topbottom task may also reflect mental rotation. Indeed, the topbottom discrimination task shows a response time function that is symmetrical about 180° of rotation, which is typical of mental rotation functions (Cooper & Shepard, 1973; Shepard & Metzler, 1971). However, a patient with right basal ganglia damage has been reported to show impairment on mental rotation tasks but normal performance on top-bottom discrimination tasks (Harris, Harris, & Caine, 2002), suggesting the effects of orientation during top-bottom discriminations are not due to mental rotation.

Furthermore, to mentally rotate an image to the depicted object's normal upright orientation through the shortest angular distance, one must know the current orientation and the normal upright orientation of the depicted object. Without this information, one could not determine the direction that results in the shortest distance. These necessary pieces of information, however, indicate that the location of the top of the depicted object must be known before mental rotation, which makes the transformation logically unnecessary (Corballis & Cullen, 1986).

There are empirical suggestions that the effects of orientation that are found during top-bottom discriminations may not reflect object processing. It has been demonstrated that the effects of orientation are larger for top response trials than for bottom response trials (Jolicoeur, Ingleton, Bartram, & Booth, 1993; McMullen & Jolicoeur, 1992). Such a finding is difficult to explain if the response time pattern reflects processing of the object stimulus and its orientation. One suggestion is that the interaction between orientation and response may be the result of a tendency to scan the display from top to bottom during the top-bottom task (Jolicoeur et al., 1993). Top-to-bottom scanning would result in dots located high on the screen being located sooner than those located further down. This dot-location effect would then combine with stimulus orientation processing effects to produce the interaction between response and orientation. Although this could be averaged out by collapsing over response, the dot effect could also be removed by subtracting out the mean response time to dots in various locations (Maki, 1986). It is also possible that the dot-location effect may be removed if the dot was presented before the object stimulus, placing the scan for the dot before the onset of the object. …

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