Browsing by Subject "Encoding"

The inferior colliculus (IC) is an auditory midbrain nucleus essential to the perception of sound frequency and the localization of sound source; yet it remains unclear how the firing rate of primate IC neurons contribute to the localization of concurrent sounds of variable sound frequencies. In this work, I extracellularly recorded the activity of 105 IC neurons while two adult macaque monkeys reported the location(s) of either a single bandpass filtered sound or two concurrent bandpass filtered sounds spatially separated by 24° and separated in sound frequency by 0.25 - 2 octaves. Monkeys performed this task well, with an accuracy of about 80% on single sound trials and about 90% on dual sound trials. The improvement in performance on dual sound trials was not explained by dual sound modulations of IC neural response functions. On dual sound trials, IC neuron receptive fields broadened, and sound frequency accounted for less variance in the dual sound response; and these changes decreased the performance of a maximum-likelihood decoder in correctly labeling the condition of a held out dual sound trial by about 20%. Overall, these results suggest that changes to the IC neural response functions elicited by the presence of a second, concurrent, sound should impair rather than facilitate the IC encoding of concurrent sounds and that an alternative explanation is required to account for monkey performance. I next investigated if recently discovered response alternations, suggested to underlie the encoding of concurrent sounds, were present in the recorded populations. These response alternations occur when an IC neuron alternates its firing rate between the rate corresponding to each component sound of a dual sound pair. These response alternations were observed in about 60% of IC neurons and their contribution to the population response remained stable across the full, 2 octave, range of frequency separations tested. Thus, response alternations are a general mechanism used by the IC to potentially facilitate the encoding of multiple sounds and these results add to a growing body of work observing response alternations across brain areas. The measurements I performed clearly indicate that neurons in the primate IC are sensitive to not only sound frequency and location but also the number of sounds in the environment. Future empirical and theoretical work is needed to elucidate how exactly these response alternations arise and are read out by downstream neurons to allow for the perception of concurrent sounds.

The ability to re-experience the past is a defining feature of episodic memory. Yet we know that even the most detailed memories are distinct from the initial experiences to which they refer. This relationship between the initial encoding and subsequent retrieval of information is central to our understanding of memory and its capacity to connect us to the past. Past research has shown that neural signatures present during perception are reactivated during later memory, but the correspondence between this reactivation and various aspects of memory function remains unclear. This dissertation attempts to connect behavioral measures of memory to the reinstatement and modification of neural information that takes place when memories are retrieved. In the first two studies reported, functional magnetic resonance imaging (fMRI) is used to assess event-specific cortical patterns from encoding that are reinstated during retrieval (encoding-retrieval similarity, ERS). Increases in this fine-grained of reinstatement are found in occipitotemporal cortex (OTC) during detailed memory for scenes (Study 1), and in the medial temporal lobes (MTL) for the recovery of relational information (Study 2). In addition to reflecting encoding-related content, retrieval is also found to strengthen previously encoded information via hippocampally-mediated mechanisms in Study 3. Together, these studies demonstrate the detailed nature of information that is recovered across varying degrees of memory and show how retrieval can also alter stored representations, emphasizing the interactive nature of memory processes.

The retina signals visual information to the brain with the parallel channels of different retinal ganglion cell (RGC) types, whose signals ultimately lead to visual perception. Between cloudy nights and sunny days, the retina must combat the trillion-fold change in mean light intensity to successfully convey visual information. Critically, the nature of both signal and noise in RGC populations is altered across this broad range of light levels, creating a rich problem of how visual messages are encoded by the retina and transmitted to the brain. This thesis addresses these topics using large-scale multielectrode array recordings of RGC populations in different light conditions. In Chapter 2, I characterize how retinal signaling is altered over a wide range of light intensities. Chapter 3 investigates how adaptation impacts visual encoding of different RGC types. My results suggest that although retinal computations change substantially over light conditions, there are some elements of visual encoding that are invariant to light adaptation. Finally, Chapter 4 examines adaptation-induced changes in the structure of correlated activity and the subsequent impact on processing retinal output. The findings of this chapter clarify the nature of RGC responses crucial for downstream readout across light levels. Overall, this work identifies aspects of RGC activity that are important for encoding visual information and decoding retinal output from starlight to sunlight.