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In four studies, we investigated how the human sensory system acts on representations of sensory information. By making use of an anisotropy in orientational sensitivity (the oblique effect), we investigated how the human visual system determines object orientation. We showed that the visual system applies an optimized strategy in selecting object features to determine global object orientation. Subsequently, we investigated how the visual system determines the orientation of an object that is not represented at the retinal level. Using Kanizsa-like square stimuli, and again making use of the oblique effect, we showed that the human visual system determines object orientation based on an object representation, and not on object features present in the retinal projection. These results seem to point to a general principle of the human sensory system, where sensory information allowing for optimal interaction with the environment (e.g. an orientation determination task) is preferred over other information. This principle holds for objects that are fully represented at the retinal level, but for objects that are represented at higher cortical areas as well, e.g. Kanizsa-like square stimuli. This latter finding implies that the visual system acts on a completed representation of an object, not on what is projected on the retina. An interesting phenomenon in the auditory domain, known as octave equivalence (e.g. Deutsch & Boulanger, 1984; Kallman, 1982), seems to suggest that auditory attention can be drawn towards a representation of an auditory object as well. The auditory system, after being presented with a tone, becomes more sensitive to frequencies similar to that tone, with respect to dissimilar frequencies, a phenomenon known as the attention band (e.g. Dai, Scharf & Buus, 1991). We used this particular aspect of auditory processing to infer how attention operates in the auditory domain. We found that attention is drawn not only to a cue frequency but to other, octave-related, frequencies that fall in the same perceptual category as well. Additionally, we showed that by merely thinking of a frequency, attention is drawn not only to the imagined frequency, but again, to octave-related frequencies as well. We discussed a possible mechanism that might give rise to this octave-effect, that reflects neural connectivity at octave-related frequencies. Such a connectivity scheme might well have evolved as a result to being exposed to environmental sounds that contain octaves of a fundamental frequency. Interestingly, spectral characterististics of human speech indeed show an overrepresentation of octave-related frequencies (Schwartz & Purves, 2003). We suggested that our results are behavioral evidence for an auditory, attentionally modulated mechanism that processes tones based on their perceptual category, rather than on absolute properties, such as frequency content. Summarizing, the chapters in this thesis point to a preference of the human sensory system to act on representations of objects, both in the visual and auditory domain |
