Why the whole is more than the sum of its parts: Salience-driven overestimation in aggregated tactile sensations

Experimental psychology often studies perception analytically, reducing its focus to minimal sensory units, such as thresholds or just noticeable differences in a single stimulus. Here, in contrast, we examine a synthetic aspect: how multiple inputs to a sensory system are aggregated into an overall percept. Participants in three experiments judged the total stimulus intensity for simultaneous electrical shocks to two digits. We tested whether the integration of component somatosensory stimuli into a total percept occurs automatically, or rather depends on the ability to consciously perceive discrepancy among components (Experiment 1), whether the discrepancy among these components influences sensitivity or/and perceptual bias in judging totals (Experiment 2), and whether the salience of each individual component stimulus affects perception of total intensity (Experiment 3). Perceptual aggregation of two simultaneous component events occurred both when participants could perceptually discriminate the two intensities, and also when they could not. Further, the actual discrepancy between the stimuli modulated both participants’ sensitivity and perceptual bias: increasing discrepancies produced a systematic and progressive overestimation of total intensity. The degree of this bias depended primarily on the salience of the stronger stimulus in the pair. Overall, our results suggest that important nonlinear mechanisms contribute to sensory aggregation. The mind aggregates component inputs into a coherent and synthetic perceptual experience in a salience-weighted fashion that is not based on simple summation of inputs.

Multisensory Versus Unisensory Integration: Contrasting Modes in the Superior Colliculus

The present study suggests that the neural computations used to integrate information from different senses are distinct from those used to integrate information from within the same sense. Using superior colliculus neurons as a model, it was found that multisensory integration of cross-modal stimulus combinations yielded responses that were significantly greater than those evoked by the best component stimulus. In contrast, unisensory integration of within-modal stimulus pairs yielded responses that were similar to or less than those evoked by the best component stimulus. This difference is exemplified by the disproportionate representations of superadditive responses during multisensory integration and the predominance of subadditive responses during unisensory integration. These observations suggest that different rules have evolved for integrating sensory information, one (unisensory) reflecting the inherent characteristics of the individual sense and, the other (multisensory), unique supramodal characteristics designed to enhance the salience of the initiating event.

Globally Perceived Directional Flow in Static Images

Visual sensitivity to spatial direction has classically been associated with motion perception. Yet humans are adept at deriving directional information in the absence of motion, as when they read maps, or follow arrows or animal tracks. Experiments are reported on the perception of parallel arrow-like forms in which a specific visual sensitivity to static direction is demonstrated. Global processing is operationally defined in terms of the relative discriminability of sets and subsets of stimulus elements; a set of parallel elements and a set in which one element is antiparallel to the rest are shown to be processed globally. The result of this global processing is a static analog of unidirectional optic flow. Global spatial direction differs fundamentally from other perceptions derived from static image processing. It involves long-range interactions in texture arrays, it does not carry information about stimulus location, and it is not reducible to the perception of component stimulus elements. Its likely function is in the construction of the layout of visual space.

Responsiveness of neurons in the hamster parabrachial nuclei to taste mixtures.

Responses from hamster parabrachial nuclei neurons to stimulation of the anterior tongue with sucrose, NaCl, HCl, quinine hydrochloride, and the six two-component mixtures of these stimuli were recorded. A cell's response to a mixture approached its response to the mixture's more effective component in the majority of cases, but was sometimes greater or smaller than this response. The best predictor of a neuron's response to a mixture, then, was its response to the mixture's more effective component. The single-component stimulus producing the maximum response was determined for each neuron and the response to this stimulus was compared with the responses evoked by the six mixtures. For 30% of the cells, a mixture elicited a response reliably, but only 1.1-2.1 times greater than the response to the best single-component stimulus. Thus, there were no neurons specialized to respond to these mixtures. The across-neuron patterns elicited by mixtures and the responses of best-stimulus classes to mixtures were studied for comparison with psychophysical data on taste mixtures. Mixtures were usually correlated with single-component stimuli in the mixture, but not with stimuli not in the mixture. In fact, five of the six mixtures fell directly between their components in a multidimensional scaling plot. In addition, a mixture was most effective in stimulating only those classes of neurons maximally stimulated by the mixture's components. These results correlate with psychophysical data suggesting that mixtures of taste stimuli evoke the same taste qualities as evoked by the mixture's components.