The target article by Jarvilehto (1998) makes the valid empirical point that perception is not the simple, serial, one-way process it is often thought to be. However, we cannot agree with the conclusion that senses are not transmitters of environmental information. Perceptual systems have developed to subserve our actions, hence actions and intentions modify sensory signals at several levels of processing, but the senses can nevertheless be studied as transmitters of information, apart from actions. It is also valid to partition a system so as to study its role in isolation from the system as a whole.
2. Modulation of incoming sensory signals, however, is well known at several levels of sensory processing. For example, the cells of the lateral geniculate nucleus (LGN) are sensitive to alertness (Livingstone & Hubel 1981), and Kosslyn et al. (1993, 1995) have found, using positron emission tomography, that visual cortex is activated by visual imagery in a manner similar to the way it is activated by actual visual stimulation through the eyes.
3. Over thirty visual areas have been identified in the thalamus, superior colliculus, and cerebral cortx, almost all of them having reciprocal feedback pathways with their feedforward areas (Felleman & Van Essen 1991). Although the existence of feedback shows that the traditional serial scheme of sensory information processing is untenable, it does not destroy the hierarchical arrangement of a sensory system; Felleman and Van Essen (1991) present social systems as examples of hierarchical organization in which feedback is necessary for their functioning no matter how independent the systems are. Feedback from neurons higher in the hierarchy is important during "bottom-up" sensory processing because it can enhance the signal-to-noise ratio. For example, feedback from the visual cortex induces correlated firing in the lateral geniculate nucleus (Sillito et al. 1994); incoming sensory signals may be amplified within V1 (Douglas et al. 1995), and feedback from the visual motion sensitive area V5 enhances directional selectivity in areas lower in the cortical hierarchy (Hup et al. 1998). These top-down effects depend on visual stimulation, however, unlike the activity in the study of Alexandrov et al. (1986).
4. A problem arises when the input organization of the sensory system is not purely bottom-up, with the information content depending on internal top-down processes such as attention, motivation or vigilance. This interference is equally problematic at the receptor level and at some later point in the sensory hierarchy, as long as it occurs before the level of classification and interpretation of sensory inputs for guiding behavior and forming conscious percepts. The findings of Kosslyn et al. (1995) are important here because they push the contents, effects and representation of imagery to an early level in the visual system.
5. On the other hand, we acknowledge the critical role of movements in information pick-up. Attention and hypothesis formation merely extend this process of obtaining information from the environment by every possible means. Sensory information is not enough to create perceptions as collections of momentary sensory input. In the input projections of the visual system, for example, every new fixation produces a distinct, highly distorted spatial projections of the same environmental scene to the surface of visual cortex (Virsu 1985) because the spatial transformation (cortical magnification factor) is highly nonlinear (Virsu & Hari 1996). Objects also look the same although they are perceived in different illuminations, sizes, velocities, and angles that change their retinal images. An additional complication is that adult sensory systems and the brain are changing continuously as a function of experience (Kaas 1995).
6. The idea of senses as static feedforward systems has to be revised towards a more dynamic and complex notion. It is difficult to conceive any fixed neural or sensory code that, apart from elementary transducer functions, could yield useful information to the organism independently of its intentions. As Gregory (1972, 1980) has pointed out, perceiving is an active process. In perception, hypotheses based on the organism's memories and intentions are tested against sensory information so as to lead to correct action. The pieces of information transmitted by the senses are only one part of the evidence, insufficient in itself. Data obtained from memory, context, interpretations, and acts are necessary additional sources of evidence used in testing hypotheses and forming perceptions. Percepts cannot be separated from the total cognitive context in which they occur, but the context can be controlled in laboratory experiments.
7. The insufficiency of sensory information for creating knowledge does not imply that the senses are not transmitters of environmental information and that the organism-environment system cannot be partitioned. Jarvilehto's thought experiment on knowledge formation without senses proves nothing because it cannot work without some receptor. A thorough study of a system requires analysis of both the whole and its parts. If the system under study were, say, the car traffic between two cities, it might be useful to study events on the road. However, this would not rule out studying car drivers and their behaviors independently for other purposes.
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