## 2 Phenomenal Consciousness Aka Sensory Experience
### Neurophysiological Difference between Feed-Forward and Feed-Back Activity in Primary Sensory Cortex  
The critical difference between feed-forward and feed-back activity in primary sensory cortex is that feed-back activity produces relatively large local field potentials (LFPs), while feed-forward activity does not. The anatomical and physiological reasons for this difference are explained below. LFPs are large extracellular electromagnetic (EM) fields, which are produced when many presynaptic axon terminals fire synchronously at chemical synapses on the apical dendrites of neocortical pyramidal cells (Figure 1). Figure 1A shows a single pyramidal neuron in the neocortex. Like all neocortical pyramidal cells, this one has a cell body shaped like a pyramid (here positioned at the bottom of Layer 5) and a single, very long, apical dendrite, running from the cell body to the surface of the brain and ending in an apical tuft in Layer 1. The voltage records in Figure 1A show what happens when incoming axons from some other area of brain (not shown) make cortico-cortical synapses at the start of the apical tuft (in the area labelled B in Figure 1A. Neurotransmitters released by the incoming axons open ion channels in the membranes of the apical dendrites and positive ions flow through these channels from the extracellular fluid to the interior of the dendrite, in accord with the electrochemical gradient between inside and outside. Figure 1: Genesis of local field potentials (LFPs). (**A**): LFPs are basically population excitatory postsynaptic potentials (EPSPs) that are generated by synaptic action on the apical dendritic tufts of pyramidal cells. When presynaptic terminals (not shown) release neurotransmitter at site B, positive ions enter the postsynaptic dendrite, leaving a transient negative ’hole’ in the extracellular fluid. To ‘complete the circuit’ an equal number of positive ions exit the postsynaptic cell nearer the cell body. This process can be modelled as a large equivalent dipole. (**B**): Cartoon showing how the negative poles of the smaller dipoles representing entry of positive ions to the postsynaptic dendrites of many synapses can summate to produce the large extracellular voltage transients seen as LFPs (green area). This summation only happens when two conditions are met: (1) many synapses are active synchronously, and (2) the postsynaptic dendrites are anatomically aligned, so that the negative poles of the small dipoles overlap spatially. This explains why large LFPs are not produced during feedforward synaptic activity. Feedforward activity goes mostly through the stellate cells in Layer 4, which do not have anatomically aligned dendrites. Large LFPs are generated only during synchronous feedback synaptic activity, as illustrated in (**B**). The inflow of positive ions leaves a transient negative 'hole' in the extracellular fluid around the synapse. Figure 1B illustrates this process diagrammatically for three adjacent dendrites, showing that the extracellular negativity becomes larger the more synapses are synchronously activated (green area in Figure 1B). As shown in Figure 1A, the influx of positive ions into each individual dendrite now causes an equivalent number of positive ions to flow out of the cell near the cell body. Thus, a transient \"equivalent dipole\" between the surface of the brain and deeper layers of extracellular fluid is generated. Again, the green area of Figure 1B shows diagrammatically how this dipole becomes much larger if a number of incoming axons fire synchronously. This effect is solely a consequence of basic neuroanatomy. It occurs because all of the apical dendrites are anatomically aligned. The critical difference between feedback and feedforward activity is also solely a consequence of neuroanatomy. It is that (as shown in Figure 1) feed-back activity occurs at Layer 1 cortico-cortical synapses, while, in contrast, feedforward thalamo-cortical connections go mainly through the stellate cells of Layer 4 [46; 47].