Song recognition learning and stimulus-specific weakening of neural responses in the avian auditory forebrain

Abstract

Learning typically increases the strength of responses and the number of neurons that respond to training stimuli. Few studies have explored representational plasticity using natural stimuli, however, leaving unknown the changes that accompany learning under more realistic conditions. Here, we examine experience-dependent plasticity in European starlings, a songbird with rich acoustic communication signals tied to robust, natural recognition behaviors. We trained starlings to recognize conspecific songs and recorded the extracellular spiking activity of single neurons in the caudomedial nidopallium (NCM), a secondary auditory forebrain region analogous to mammalian auditory cortex. Training induced a stimulus-specific weakening of the neural responses (lower spike rates) to the learned songs, whereas the population continued to respond robustly to unfamiliar songs. Additional experiments rule out stimulus-specific adaptation and general biases for novel stimuli as explanations of these effects. Instead, the results indicate that associative learning leads to single neuron responses in which both irrelevant and unfamiliar stimuli elicit more robust responses than behaviorally relevant natural stimuli. Detailed analyses of these effects at a finer temporal scale point to changes in the number of motifs eliciting excitatory responses above a neuron's spontaneous discharge rate. These results show a novel form of experience-dependent plasticity in the auditory forebrain that is tied to associative learning and in which the overall strength of responses is inversely related to learned behavioral significance.

Fig. 1.

Auditory diagram and song recognition training. A: schematic of the songbird auditory forebrain. Ov, nucleus ovoidalis; L, field L; NCM, caudomedial nidopallium; CM, caudal mesopallium. B: schematic of the operant panel used for song recognition learning. C: acquisition curve showing the mean ± SE performance (d-prime) over the 1st 100 blocks of song recognition training for 9 starlings, 100 trials per block.

Fig. 2.

Recording location. Parasagittal section of a starling brain showing an electrode tract and small fiducial electrolytic lesion in NCM. The arrowheads mark the electrode tract and the lesion. CM, caudal mesopallium; Hp, hippocampus; NCM, caudomedial nidopallium.

Fig. 3.

Preference for unfamiliar songs in single neurons. A: spectrogram, peristimulus time histogram (PSTH), and raster plot for responses of a single neuron to 5 repetitions of the 3 learned and (B) 3 unfamiliar songs that elicited the strongest mean firing rates from this neuron. Spikes are binned in 20-ms bins for the PSTH. C: sample trace showing raw voltage recorded during the 1st repetition of the bottom song. Zero marks the time of stimulus onset. D: overlay of spike waveforms with the mean (gray line). E: distribution of interspike intervals. Capped at 100 ms to show values near 0. F: zoomed in raster, PSTH, and spectrogram for the 3 motifs underlined in the bottom song in B. The separate lines show the start and stop of the different motifs. The mean spontaneous firing rate of this neuron was 5.46 spikes/s. This neuron was recorded at a depth of 3,020 μm.

Fig. 4.

Preference for unfamiliar songs in single neurons. As in Fig. 3 for a different NCM neuron. The mean spontaneous firing rate of this neuron was 0.13 spikes/s. This neuron was recorded at a depth of 2,920 μm. In D, there is some variability in spike height caused by an improvement in isolation during recording as spike height increased.

Fig. 5.

Preference for unfamiliar songs in NCM neurons. A: bar graph showing the bias values for neurons from different depth quartiles. The range of the quartiles from dorsal to ventral was 1,090–1,870 (n = 21), 1,871–2,580 (n = 24), 2,581–3,065 (n = 24), and 3,066–4,091 μm (n = 24), measured from the surface of the brain. *Bias values significantly different from 0. B: distribution of bias values for 48 ventral NCM neurons. Bias values >0 indicate neurons that responded higher to unfamiliar songs, whereas bias values <0 indicate neurons that responded higher to learned songs. C: distribution of bias values for 45 dorsal NCM neurons. D: bar graph showing the z-scores of the firing rates to the Go, No-go, and unfamiliar songs for 48 ventral NCM neurons. E: bar graph showing the z-scores of the firing rates to the Go, No-go, and unfamiliar songs for 45 dorsal NCM neurons. *P < 0.05. NS, P > 0.05. Error bars show SE.

Fig. 6.

Song exposure does not cause weakened responses to learned songs in NCM. A: diagram showing modified recognition-training procedure where the starlings alternated between 1-h training and passive listening blocks. B: mean acquisition curve for the song recognition learned in the training block for 3 starlings. D′ values were calculated over blocks of 100 trials. Data are shown for the 1st 97 blocks—not the entirety of each starling's training. Error bars show SE. C: bar graph showing the z-scores of the firing rates to learned, unfamiliar, and passively heard songs for 26 NCM neurons. *P < 0.05. NS, P > 0.05. Error bars show SE.

Fig. 7.

No evidence for firing rate adaptation during recording experiments. Change in firing rate across 5 repetitions in the 1st neuron recorded in each starling. For visualization, the firing rates of each neuron have the mean firing rate on the 1st repetition subtracted. Each point is the mean firing rate across all song stimuli.

Fig. 8.

Motif-level contributions to song-level effects. A and B: scatter plots showing the relationship between the percentages of motifs from learned and unfamiliar songs that elicited significant (A) excitatory or (B) suppressive responses in each neuron. Significant excitatory and suppressive responses are defined as any response an SD above or below the spontaneous rate, respectively. Each circle corresponds to 1 neuron. The gray cross in each plot shows the mean and SE.