A hidden ambiguity of the term "feedback" in its use as an explanatory mechanism for psychophysical visual phenomena.

Abstract

Over the last decades many researchers have used concepts like “feedback,” “reentrance,” “backpropagation,” “top–down (modulation),” or “reverse hierarchy” to specify the mechanisms that underlie various visual phenomena (e.g., Di Lollo et al., 2000; Lamme and Roelfsema, 2000; Pascual-Leone and Walsh, 2001; Super et al., 2001; Ro et al., 2003; Ahissar and Hochstein, 2004; Bar et al., 2006; Fahrenfort et al., 2007; Koivisto, 2012). An incomplete list of these phenomena includes visual (object substitution) masking, shape discrimination, illusory contours, illusory motion, priming effects, etc. Empirical evidence or theoretical argumentation in favor of the suggested mechanismic explanations mainly consists in finding or postulating an association between a temporally delayed, secondary activition of lower level neural units with correct reports of target stimuli, even though the higher level neural units in the processing hierarchy were already activated earlier. On that basis, feedforward processing has been argued to be insufficient for target perception. However, in most of the studies the relative temporal order of activity at different levels alone is taken as proof of reentrant modulation without precisely measuring the neural sources of this top–down effect. In principle, it is equally possible that the source of the higher level activity from which the top–down signals are sent back to earlier feature-encoding neural units (i) is specifically linked to those features by virtue of constituting the higher level nodes associated with specific attributes of the target stimulus (thus mediating feature-binding for object integration) or (ii) is not specifically linked in this manner. In the latter case, the source of top–down modulation may be the result of the arousal or alerting boost triggered by the target stimulus via feedforward collateral activation of subcortical reticulo-thalamic units, which in turn is followed by the cortical spread of the thalamocortical activation, including the downpropagation of the non-specific wave of modulation to the early cortical areas. The non-specific system functions include arousal, attentional modulation, intercortical synchronization of neural activity, bringing the preconsciously processed specific content to awareness, “event-holding” the content in working memory, and alerting subjects to newly appearing objects and changes (Magoun, 1958; Purpura, 1970; Purpura and Schiff, 1997; Jones, 2001; Llinas and Ribary, 2001; Van der Werf et al., 2002; Ribary, 2005; Schiff et al., 2013; Saalmann, 2014). This non-specific system (NSP) targets layer-1 apical dendrites of the layer-5 and -6 pyramidal neurons. But since NSP-modulation is directed at the cortical neurons with specific representational functions, its function may go unacknowledged because the cortical units, when activated by NSP-modulation, can produce content-specific subjective effects misleading us to believe that the entire process has been specific throughout.