Blog – Page 614

In the last thread, someone asked what exactly is it about consciousness that illusionists say is illusory?

One quick answer is that for illusionists, the properties people see in experience that incline us to think that consciousness is a metaphysically hard problem, are what’s illusory. In weak illusionism, the properties aren’t what they seem. In the strong version, which is usually what “illusionism” refers to, they don’t exist at all. But what exactly are these properties?

I’m a functionalist, someone who sees conscious experiences, and mental states overall, as more about what they do, the causal roles they play, than about any particular substance or constitution. It’s a view that I think provides a necessary explanatory layer between the mental and the physical, and so sees no barrier in principle to a full understanding of the relationship between them.

Some aspects of this complex process, such as integration at the level of individual dendritic branches, have been extensively studied. But other aspects, such as how inputs from multiple branches are combined, and the kinetics of that integration have not been systematically examined. Using a 3D digital holographic photolysis technique to overcome the challenges posed by the complexities of the 3D anatomy of the dendritic arbor of CA1 pyramidal neurons for conventional photolysis, we show that integration on a single dendrite is fundamentally different from that on multiple dendrites. Multibranch integration occurring at oblique and basal dendrites allows somatic action potential firing of the cell to faithfully follow the driving stimuli over a significantly wider frequency range than what is possible with single branch integration. However, multibranch integration requires greater input strength to drive the somatic action potentials. This tradeoff between sensitivity and temporal precision may explain the puzzling report of the predominance of multibranch, rather than single branch, integration from in vivo recordings during presentation of visual stimuli.

Individual thin dendritic branches are fundamental functional units in the nervous system (Branco and Hausser, 2011). Experimental data support the concept that they can operate as quasi-independent processing and signaling units capable of non-linear behavior (Mel, 1993; Wei et al., 2001). In combination with their parent dendritic branches, these thin distal dendrites can function in two distinct modes (Gasparini and Magee, 2006; Katz et al., 2009). If distributed synaptic inputs arrive on multiple distal branches, the depolarization on each branch may be below the threshold for recruiting local active conductances in a regenerative manner and yet be sufficient to trigger a somatic sodium spike. This is sometimes referred to as the traditional “integrate and fire” model (Abbott, 1999), the “synaptic democracy” model (Yuste, 2011), and the “global” model of integration.