Handle peak_sign for quality metrics in _handle_backward_compatibility_on_load (#4481)

Author: chrishalcrow

src/spikeinterface/metrics/quality/quality_metrics.py

@@ -77,6 +77,20 @@ def _handle_backward_compatibility_on_load(self):
             if "peak_sign" in self.params["metric_params"]["amplitude_median"]:
                 del self.params["metric_params"]["amplitude_median"]["peak_sign"]
 
+        # TODO: update this once `main_channel_index` PR is merged
+        # global peak_sign used to find appropriate channels for pca metric computation
+        # If not found, use a "peak_sign" set by any metric
+        global_peak_sign_from_params = self.params.get("peak_sign")
+        if global_peak_sign_from_params is None:
+            for metric_params in self.params["metric_params"].values():
+                if "peak_sign" in metric_params:
+                    global_peak_sign_from_params = metric_params["peak_sign"]
+                    break
+            # If still not found, use <0.104.0 default, "neg"
+            if global_peak_sign_from_params is None:
+                global_peak_sign_from_params = "neg"
+            self.params["peak_sign"] = global_peak_sign_from_params
+
     def _set_params(
         self,
         metric_names: list[str] | None = None,