In this target article we question the assumption that perception isdivided into separate domains of vision, hearing, touch, taste, andsmell. We review implications of this assumption for theories ofperception and for our understanding of ambient energy arrays (e.g.,the optic and acoustic arrays) that are available to perceptualsystems. We analyze three hypotheses about relations between ambientarrays and physical reality: (1) that there is an ambiguous relationbetween ambient energy arrays and physical reality, (2) that thereis a unique relation between individual energy arrays and physicalreality, and (3) that there is a redundant but unambiguous relation,within or across arrays, between energy arrays and physical reality.This is followed by a review of the physics of motion, focusing onthe existence and status of referents for physical motion. Ourreview indicates that it is not possible, in principle, for there tobe a unique relation between physical motion and the structure ofindividual energy arrays. We argue that physical motion relative todifferent referents is specified only in the globalarray, which consists of higher-order relations acrossdifferent forms of energy. The existence of specificity in theglobal array is consistent with the idea of direct perception, andso poses a challenge to traditional, inference-based theories ofperception and cognition. However, it also presents a challenge tomuch of the ecological approach to perception and action, which hasaccepted the assumption of separate senses.

Based on studies with infants, we expand on Stoffregen & Bardy'sexplanation of perceptual motor errors, given the global array.Information pick-up from the global array is not sufficient withoutadequate exploratory movements and learning to support perceptuallyguided activity.

The global array may be a useful concept in studying behavior in acomplex environment, especially in the context of dynamical systemstheory. However, Stoffregen & Bardy's arguments are weakened bythe conflation of sensation and perception, and by the lack ofevidence for synergy between stimulus energy arrays; strong evidenceplaces the convergence of sensory stimuli inside the head.

Stoffregen & Bardy's target article is based on the assumptionthat our senses' ultimate purpose is to provide us with perfectinformation about the outside world. We argue that it is often moreimportant that information be available quickly than that it beperfect. Consequently our nervous system processes different aspectsof information about our surrounding as separately as possible. Theseparation is not between the senses, but between separate aspectsof our surrounding. This results in inconsistencies betweenjudgments: sometimes because different frames of reference are used.Such inconsistencies are fundamental to the way the information ispicked up, however, and hence cannot be avoided with clearerinstructions to the subjects.

Stoffregen & Bardy suggest that the global array provides thespecification that is lacking when senses are considered inisolation. This seems to beg the question of the minimum number ofsenses in a global array. Individuals with sensory loss manage withfewer senses, and humans manage with fewer than electric fish; sospecification, if it exists, cannot require all possible senses.

Stoffregen & Bardy propose an absolutist transmodal arraystructure model, intended to displace models of specification in allexisting perceptual theories. Absolute specification of worldstructure in array structures, either unimodal or transmodal, maynot be provable, but might be falsifiable. Absolute specification,moreover, may not be a necessary postulate in an ecological approachto understanding perception-action.

Is there any ecological purpose in assuming that perception foraction exists only through a global array of energy? UnlikeStoffregen & Bardy, who assume that behavior consists ofmovements, we would argue that behavior consists of a stablecoupling between perception and action achieved through experiencein an adaptive context. Determining target position in an aimingmanual task and temporal control of impact movement illustrate thatpatterns of energy used for action are task-dependent.

Stoffregen & Bardy argue that unimodal invariants do not exist,and that only “amodal” invariants are possible. But they confuse twoseparate issues. Amodal invariants, we argue, do indeed exist tospecify features of the environment, but not even an amodalinvariant, in isolation, could specify their “presence” or“reality.”

The information required for perception may be available in theenergy arrays that stimulate sensory organs but in a form notdirectly suitable for the planning and execution of the organism'sactions in the environment. The requisite form of information isobtained, with no loss of adequate perception, by representation ofsensory stimuli in frames of reference determined by internalcontrol signals producing actions. This process seems evolutionarilyadvantageous but makes perception essentially non-direct, regardlessof the degree of intra- or inter-modal sensory integration.

We find Stoffregen & Bardy's argument that the senses are unitedand that specificity exists within the global array compelling.However, this view is not entirely new and research on thedevelopment and the origins of perception in infancy, inspired byGibson's ecological perspective, also supports their claims. Theinclusion of this developmental research will strengthen andchallenge some of Stoffregen & Bardy's views.

We believe that the task goal in voluntary movements provides meaningto existing information sources in the environment and determines,in a dynamic way, the use and relative importance of these differentsources. This task-centered meaning bridges the apparent controversybetween what information is available in principle (i.e.,specification), and what information is perceived.

Claims made in the target article that information-that-specifies iscontained in relations among structured energy arrays, considered aswholes, may well be true, but claims that information-that-specifiesis only contained in irreducible relations among structured energyarrays, considered as wholes, fail to persuade, at least in the caseof the blind.

Stoffregen & Bardy suggest that the senses are not separable.However, they have a philosophical approach rather than using directevidence that the nervous system analyzes sensory information in ahighly flexible manner.

The arguments of Stoffregen & Bardy for studying perception basedon the global array are intriguing. This theory can be examined innonhuman species using nonverbal tasks. We examine how monkeysmaster a skill that incorporates a two-dimensional/three-dimensionalinterface. We feel this provides excellent support for Stoffregen& Bardy's theory.

Stoffregen & Bardy reject the likelihood that infants aresensitive to the global array, implying that intersensoryintegration is not possible in early development. We argue thatinfants are sensitive to unimodal arrays and are able to integratethem through the active participation of their nervous system andthat the observed developmental changes are due to experience andbrain development.

This commentary discusses a strategy by which investigators canexamine whether observers perceive properties of the global arrayindependently of properties in individual sensory arrays. Researchshowing that perception of complex relationships appears to beindependent of the perception of individual components isconsidered. Ashby and Townsend's (1986) methods for identifyingperceptual independence are important tools for studying the globalarray.

James Gibson's theory of information, as specific to (but notnecessarily “like”) its sources, is especially indebted to the studyof occlusion for its core examples. In occlusion, one is “amodally”aware of hidden surfaces. Is this still too related to one modalityto count as a good case for Stoffregen & Bardy?

Stoffregen & Bardy's arguments against separation of the sensesfail to consider the functional differences between the kinds ofinformation potentially available in the structured energy arraysthat correspond to the traditional senses. Since mostperception/action research pursues a strategy of informationperturbation presupposing differential contributions from thevarious ambient arrays, the global array hypothesis can only beextended and tested by analyses that consider the functional aspectsalong which the senses can, in fact, be separated.

There is a discrepancy between Stoffregen & Bardy's concept withexperimental work on human self-motion perception. We suggest analternative: (1) higher brain centers are informed by a givensensory cue in a direct and rapid way (direct perception), and (2)this information is then used to prime and shape a more complexmechanism that usually involves several cues and processing steps(inferential).

Before one can talk about global arrays and multimodal detection, onemust be clear about the concept of information: How is it differentfrom energy and how is it detected? And can it come to specify aneeded movement? We consider these issues in our commentary.