Part 1: Last week we examined the hierarchical organization of neurons in the human visual system that allow us to generate complex visual representations by combining inputs from simpler representations. Another domain of the brain in which we observe this hierarchical organization is the auditory system. In your ears, there exists a spiral-shaped structure called the Cochlea, which is known to contain hair cells that vibrate in response to different pitches (aka frequencies) of sound. These cells then project to the primary auditory cortex (A1), where there is a “tonotopic” organization, meaning that neurons in A1 are arranged in order of their pitch-preference, where neurons that respond to lower pitches are more anterior (towards the front of the brain) and neurons that respond to higher pitches are located more posterior (towards the back of the brain).The primary auditory cortex is located on the upper side of the temporal lobe. We hypothesize that in some subsequent auditory area, there exist cells that respond most strongly to chords (combinations of pitches that are played at the same time). Draw a wiring diagram of a hypothetical cell that encodes the C minor chord, comprised of the notes C, E flat, and G. (Hint: You can assume there are 3 neurons in A1 which encode the frequencies corresponding to C, E flat, and G, respectively).
Part 2: Minor chords, which are defined by a particular interval size between the notes, are often characterized as sounding “sad”. In fact, many people can detect a minor chord by this sad sound, even if they are unable to identify which minor chord they are hearing! Now, let’s draw a hierarchical wiring diagram that explains how people are able to identify minor chords, even if they cannot tell which one.
Part 3: Using the terms invariance and selectivity, explain how your hierarchical diagram gives rise to the ability to identify minor chords, regardless of the root note.
